Azure Communication Services Call Latency

Question

Azure Communication Services Call Latency

Dave Weisberg 0

We are experiencing longer than normal latency using Azure Communication Services when connecting two PTSN participants.

We are using AzureCommunicationServices.CallAutomation NuGet package v1.2.0 and the CreateCallAsync method to establish a call to a single PTSN participant. And then we subsequently use the AddParticipantAsync method to add a second PTSN participant to the Session.

The latency between these two participants is typically between 1 - 2 seconds as measured by using a voice echo test phone number found here https://testcall.com/804-222-1111/. If I call the voice echo test phone number directly from my cell phone, I get about 100-200 mSec of latency. So the latency is significantly increased using the Azure Communication Services service.

My end users are complaining that the latency is intolerable for their needs, and they quite often will not use the service because it is so disruptive to their work.

Why is this latency so long, and is there any way to reduce it?

Bhargavi Naragani 3,165 Reputation points Microsoft External Staff

2025-04-28T05:03:13.69+00:00
Hi @Dave Weisberg,

In addition to Sina Salam response,

The latency you are experiencing when connecting PSTN participants using Azure Communication Services could be attributed to several factors. The call setup process involves multiple steps and messages exchanged between the signaling layer and media transport, which can introduce delays.

Check the network stability and bandwidth at both ends of the call. High packet loss or jitter can affect call quality significantly. If the PSTN participants are in different geographic locations, the distance can add to latency. You might want to verify that both callers have the least amount of network hops possible.

Since you've already observed latency during your echo test calls, consider running more detailed network diagnostics to measure latency during actual calls. Ensure that there's proper Quality of Service (QoS) configured for your Azure setup to prioritize voice traffic.

You might want to check if there are any configurations within your CallAutomation setup that could be optimized.

Moreover, I would request you to provide below details to help narrow down potential causes:

What geographic locations are the PSTN participants in?

Are there network appliances (like firewalls or routers) that may be influencing latency at either side?

Are both participants on stable, high-quality broadband connections?

Have you noticed if latency varies at different times of the day?

Troubleshoot Web VoIP Quality
Addressing Delay Issues in Voice
Manage Call Quality
Laxman Reddy Revuri 4,000 Reputation points Microsoft External Staff

2025-04-28T22:34:07.3933333+00:00

Hi @Dave Weisberg,
We haven’t heard from you on the last response and was just checking back to see if you have a resolution yet. In case if you have any resolution please do share that same with the community as it can be helpful to others. Otherwise, will respond with more details and we will try to help.
Dave Weisberg 0 Reputation points

2025-04-29T01:58:15.5366667+00:00
Hello Bhargavi Naragani,

Thank you so much for your response. I appreciate your feedback and will attempt to answer your questions.

My call setup process is as follows. Please let me know if my setup can be improved.

private void StartCall(string targetPhoneNumber, string acsPhoneNumber) { var target = new PhoneNumberIdentifier(targetPhoneNumber); // PTSN phone number var caller = new PhoneNumberIdentifier(acsPhoneNumber); // Azure Communication Service Phone Number var callInvite = new CallInvite(target, caller); CreateCallOptions options = new CreateCallOptions(callInvite, new Uri(callback)); CreateCallResult createCallResult = await client.CreateCallAsync(options); _CallId = createCallResult.CallConnectionProperties.CallConnectionId; } private void AddCaller(string targetPhoneNumber, string acsPhoneNumber) ( var existingConnection = client.GetCallConnection(_CallId); var target = new PhoneNumberIdentifier(targetPhoneNumber); // PTSN phone number var caller = new PhoneNumberIdentifier(acsPhoneNumber); // Azure Communication Service Phone Number var callInvite = new CallInvite(target, caller); AddParticipantOptions participantOptions = new AddParticipantOptions(callInvite); AddParticipantResult result = await existingConnection.AddParticipantAsync(participantOptions); }

I'm unaware of the interactions between the signaling layer and media transport. Do you know of any documentation that can help point me in the right direction to optimize this?

If you have any docs on how to configure proper QoS for my Azure setup to prioritize voice traffic, that would be great. I assumed that Azure Communication Services would handle network operations, so I haven't looked into this.

Both calls are established from Azure Communication Services to PTSN telephones. I'm not sure how I would go about checking the network latency at both ends? A telephone cannot provide network statistics and neither participant even requires a network connection.

The geographic locations of the participants are somewhere in the United States, so there could be as much as 3500 miles between participants. Is there anything I can do to reduce latency due to distance? Maybe there is a relationship between the Azure Communication Services configuration and the location of the participants? Can you provide any insight?

I don't know of any network appliances between the participant's PTSN telephone and Azure Communication Services. Participants are using telephones connected to public telephone services.

Neither participant has a broadband connection. The calls are being received on their PTSN telephones, so I'm assuming the quality of their broadband connection (if they had a broadband connection) doesn't apply.

I have noticed no change in latency during different times of the day.

Look forward to your reply!

Thank you!

-Dave
Bhargavi Naragani 3,165 Reputation points Microsoft External Staff

2025-04-29T08:15:50.97+00:00
Hi @Dave Weisberg

To optimize your Azure Communication Services call setup, you can refer to several aspects of network quality and configuration. Here are some suggestions based on your setup and the context provided:

Device and network issues are considered external problems from the perspective of the ACS Calling SDK. Your application should integrate the User Facing Diagnostics to monitor device and network issues and display warning messages accordingly. In this way, users are aware of the issue and can troubleshoot on their own.

Sometimes the delay you're seeing when setting up calls can come from network issues like slow speeds, high round-trip times, or packet loss. You can use the Azure Communication Services SDK to measure how long it takes for a call to connect, which can help pinpoint where the slowdown is happening.

If your network is handling a lot of different types of traffic, setting up QoS can ensure voice calls are prioritized over things like file transfers or streaming. This can make a big difference in how smooth your calls are, especially in busy network environments. Improve and manage call quality

Even though your users are on PSTN phones (so their broadband connection doesn’t apply), the overall network quality on your side like latency and packet loss between Azure and the PSTN gateway, still plays a big role in call quality. It's important to follow Azure's recommended network configurations to keep things running smoothly. Network recommendations

While you can't change where people are located, you can choose to host your Azure Communication Services in a region that’s geographically closer to your users. That can help reduce latency just by shortening the distance the media has to travel.

Azure offers tools like the Network Diagnostic tool that can check if your current setup is optimized for voice and video. It’s a helpful way to find out if your network is contributing to call issues or if everything looks good on that front.

Kindly refer to the below documentations for better understanding:
The user experiences delays during the call
Troubleshoot VoIP call quality
The call setup takes too long

Hope this information is helpful, let me know if you have any further queries.
Prabhavathi Manchala 1,050 Reputation points Microsoft External Staff

2025-04-29T21:10:54.87+00:00

Hi @Dave Weisberg,

We haven’t heard from you on the last response and was just checking back to see if you have a resolution yet. In case if you have any resolution please do share that same with the community as it can be helpful to others. Otherwise, will respond with more details and we will try to help.
Dave Weisberg 0 Reputation points

2025-04-30T15:33:23.26+00:00
I'm not clear on how using User Facing Diagnostics will help improve my application. Since each participant is on a PTSN telephone, there are no adjustments or network improvements that can be made by them to improve the call quality. There are no participants on these calls using an IP-based network. Letting them know that the network is currently slow will not be helpful.

"You can use the Azure Communication Services SDK to measure how long it takes for a call to connect, which can help pinpoint where the slowdown is happening." Can you provide an example or some documentation that can help me use the SDK to pinpoint where the slowdown is happening and how to correct it by measuring how long it takes to connect a call?

Since each participant is on a PTSN telephone, there are no QoS or other network settings that can be adjusted by them. They are not connected to an IP network. The only participants on these calls are PTSN telephone users.

My network connection initiates the call using the REST API that I described, and nothing else. If I improve the bandwidth at which I could initiate the call and add a participant, will that improve the call quality for my PTSN participants?

Can you provide any documentation or insight on how to change the region of my Azure Comm Services? Currently, my Location for ACS is listed as "Global" and my Data Location is "United States." Since all my calls are in the United States, I'm not sure how to improve this.

All tests pass when I run the Network Diagnostic Tool (https://azurecommdiagnostics.net/), but again, I'm not sure how my network connectivity is relavent to the call quality of my PTSN users.
Sampath 2,345 Reputation points Microsoft External Staff

2025-05-01T15:04:31.6933333+00:00
Hello @Dave Weisberg ,

To identify where latency occurs, you can use timestamps around SDK methods to Improve and manage call quality:

var startCreate = DateTime.UtcNow; var endCreate = DateTime.UtcNow; Console.WriteLine($"CreateCallAsync latency: {(endCreate - startCreate).TotalMilliseconds} ms");

1 answer

Your answer

Bhargavi Naragani 3,165 Reputation points Microsoft External Staff

2025-04-28T05:03:13.69+00:00

Hi @Dave Weisberg,

In addition to Sina Salam response,

The latency you are experiencing when connecting PSTN participants using Azure Communication Services could be attributed to several factors. The call setup process involves multiple steps and messages exchanged between the signaling layer and media transport, which can introduce delays.

Check the network stability and bandwidth at both ends of the call. High packet loss or jitter can affect call quality significantly. If the PSTN participants are in different geographic locations, the distance can add to latency. You might want to verify that both callers have the least amount of network hops possible.

Since you've already observed latency during your echo test calls, consider running more detailed network diagnostics to measure latency during actual calls. Ensure that there's proper Quality of Service (QoS) configured for your Azure setup to prioritize voice traffic.

You might want to check if there are any configurations within your CallAutomation setup that could be optimized.

Moreover, I would request you to provide below details to help narrow down potential causes:

What geographic locations are the PSTN participants in?

Are there network appliances (like firewalls or routers) that may be influencing latency at either side?

Are both participants on stable, high-quality broadband connections?

Have you noticed if latency varies at different times of the day?

Troubleshoot Web VoIP Quality
Addressing Delay Issues in Voice
Manage Call Quality
Laxman Reddy Revuri 4,000 Reputation points Microsoft External Staff

2025-04-28T22:34:07.3933333+00:00

Hi @Dave Weisberg,
We haven’t heard from you on the last response and was just checking back to see if you have a resolution yet. In case if you have any resolution please do share that same with the community as it can be helpful to others. Otherwise, will respond with more details and we will try to help.
Prabhavathi Manchala 1,050 Reputation points Microsoft External Staff

2025-04-29T21:10:54.87+00:00

Hi @Dave Weisberg,

We haven’t heard from you on the last response and was just checking back to see if you have a resolution yet. In case if you have any resolution please do share that same with the community as it can be helpful to others. Otherwise, will respond with more details and we will try to help.
Dave Weisberg 0 Reputation points

2025-04-30T15:33:23.26+00:00

I'm not clear on how using User Facing Diagnostics will help improve my application. Since each participant is on a PTSN telephone, there are no adjustments or network improvements that can be made by them to improve the call quality. There are no participants on these calls using an IP-based network. Letting them know that the network is currently slow will not be helpful.

"You can use the Azure Communication Services SDK to measure how long it takes for a call to connect, which can help pinpoint where the slowdown is happening." Can you provide an example or some documentation that can help me use the SDK to pinpoint where the slowdown is happening and how to correct it by measuring how long it takes to connect a call?

Since each participant is on a PTSN telephone, there are no QoS or other network settings that can be adjusted by them. They are not connected to an IP network. The only participants on these calls are PTSN telephone users.

My network connection initiates the call using the REST API that I described, and nothing else. If I improve the bandwidth at which I could initiate the call and add a participant, will that improve the call quality for my PTSN participants?

Can you provide any documentation or insight on how to change the region of my Azure Comm Services? Currently, my Location for ACS is listed as "Global" and my Data Location is "United States." Since all my calls are in the United States, I'm not sure how to improve this.

All tests pass when I run the Network Diagnostic Tool (https://azurecommdiagnostics.net/), but again, I'm not sure how my network connectivity is relavent to the call quality of my PTSN users.
Sampath 2,345 Reputation points Microsoft External Staff

2025-05-01T15:04:31.6933333+00:00

Hello @Dave Weisberg ,

To identify where latency occurs, you can use timestamps around SDK methods to Improve and manage call quality:

var startCreate = DateTime.UtcNow; var endCreate = DateTime.UtcNow; Console.WriteLine($"CreateCallAsync latency: {(endCreate - startCreate).TotalMilliseconds} ms");

Answer 1

Hello Dave Weisberg,

Welcome to the Microsoft Q&A and thank you for posting your questions here.

I understand that you're experiencing Azure Communication Services.

Sorry about that, latency can be influenced by several factors, including network conditions, packet loss, jitter, and the technologies used in the call. Please, kindly review the links provided here to read more, troubleshoot and potentially reduce the latency:

Further to the above while reviewing the threads, I discovered that most of the prescribed solution offered general VoIP troubleshooting steps, mostly applicable to VoIP-to-VoIP calls, or calls involving browsers or apps, not PSTN endpoints.

Let address in more technical method:

The architecture of PSTN-to-PSTN calling via ACS.
The media path traversal and routing delays introduced by Azure infrastructure.
Known latency behavior or SLAs for PSTN bridging through ACS.

With following steps:

Step 1: Use Azure Communication Services Call Diagnostics and Azure Monitor integration for telemetry to monitor and troubleshoot voice and video calls for:

initialCallSetupDurationMs
callConnectionStartToMediaStartMs
postDialDelay
roundTripTimeMs (if available on server-side)
callTransferDelay

If you're not capturing these yet, enable Diagnostic Logging for ACS with Azure Monitor and Azure Log Analytics to do so - https://learn.microsoft.com/en-us/azure/communication-services/concepts/voice-video-calling/manage-call-quality

Step 2: ACS currently does not auto-detect optimal media relay region based on participant phone numbers.

Use:

CreateCallOptions options = new CreateCallOptions(invite, new Uri(callback))
{
    AzureRegion = CommunicationCallingServerRegion.EastUS // Example region
};

Try creating calls in regions geographically close to one or both PSTN participants. - https://learn.microsoft.com/en-us/dotnet/api/azure.communication.callautomation.createcalloptions.azureregion

Step 3: Azure Communication Services uses carrier partners to terminate PSTN calls. Therefore, Latency could be introduced in:

Carrier hops
Media relay between legs
Geographic mismatches in call setup and media paths

This is opaque to users, but Microsoft support can help.

So, open a Support Ticket via your Portal to request:

The actual routing path and regions used for media relay.
Check if media is hairpinned via a central region (e.g., West US) even if users are on the East Coast.

Step 4: Instead of PSTN-to-PSTN, try to test alternative architectures. For an example:

PSTN > ACS Client and ACS Client > PSTN bridge (e.g., via a web app or bot).

This introduces more control over media and diagnostics, and it can also optimize with Direct Routing via Azure Operator Connect if available.

Step 5: Also, you can try to replicate the call using:

Twilio, Vonage, or SignalWire, all of which provide PSTN-to-PSTN bridge services.
Compare latency for the same endpoints and call path.
If ACS latency is consistently worse, consider raising this with Microsoft Azure’s product team.

This is final Suggestion:

Use AzureRegion to bring ACS closer to participants it can minimizes routing time.
Enable ACS diagnostics + Azure Monitor it can also provide hard data.
Use ACS + Client + PSTN instead of PSTN-to-PSTN to allows media control
Open MS Support Ticket via Priority Customer Support (PCS) to inspect media path to get visibility into peering delays.

I hope this is helpful! Do not hesitate to let me know if you have any other questions or clarifications.

Please don't forget to close up the thread here by upvoting and accept it as an answer if it is helpful.

Dave Weisberg 0 Reputation points

2025-05-02T02:57:55.3566667+00:00

Hi Sina Salam,

Thank you for the response. I will attempt to implement your suggestions.

Do you have any documentation, information, or examples on how to set up PSTN > ACS Client and ACS Client > PSTN bridge?

Thank you,

Dave
Sampath 2,345 Reputation points Microsoft External Staff

2025-05-02T04:25:42.9533333+00:00

Hello @Dave Weisberg ,

Below are resources on setting up PSTN > ACS Client and ACS Client > PSTN bridge:

Quickstart guide for calling a telephone number using Azure Communication Services—This provides step-by-step instructions on how to add PSTN calling to your app.

PSTN integration concepts for Azure Communication Services—This explains key telephony concepts and capabilities, including Azure direct routing.

Azure Communication Services PSTN Calling GitHub repository—This contains sample code and setup instructions for implementing PSTN calling with Azure Communication Services.

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Azure Communication Services Call Latency

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