How to transfer a "anonymous" caller in Azure Communication Services ?

Sameer 40

Our application is using Azure Communication Services (ACS) to handle incoming telephone calls. In some cases, we need to transfer the call to another external number. When the caller's number is "anonymous", ACS responds with a 400 error for transferToParticipant

Error

Transferring participant ("SOME_CALL_CONNECTION_ID") with target: {:phone_number, "+1234567890"} and transferee: {:phone_number, "anonymous"} and options: %{}

ACS request completed unsuccessfully: [400] "{\"error\":{\"code\":\"8523\",\"message\":\"Transferee is required for transfering in a call with more than 2 participants\"}}" for https://some-acs-resource-name.communication.azure.com/calling/callConnections/SOME_CALL_CONNECTION_ID:transferToParticipant?api-version=2024-11-15-preview

Question

Whats a valid identifier that can be used for the transferee who ACS has marked as "anonymous" ?

Shree Hima Bindu Maganti 4,925 Reputation points Microsoft External Staff Moderator

2025-03-18T15:53:00.3233333+00:00

Hi @Sameer,
In Azure Communication Services (ACS), transferring a call with an anonymous participant can result in issues like the 400 error you encountered. This error indicates that a valid transferee is required when transferring a call with more than two participants, and the anonymous caller does not provide a valid identifier for this.

To successfully transfer a call with an anonymous participant, you may need to use a workaround. One approach is to establish a valid connection with a known participant or number before attempting the transfer. This involves using the transfer API with a valid target participant who is not anonymous.

Unfortunately, there is no direct identifier for the anonymous caller as a transferee in the transfer process. You may need to handle the anonymous caller differently, such as providing them with a known number or participant to transfer to.
let me know if you any further assistances.

Azure Communication Services direct routing: SIP protocol details

Transfer calls
Sameer 40 Reputation points

2025-03-18T16:24:24.44+00:00

This error indicates that a valid transferee is required when transferring a call with more than two participant

Just in case it matters, the other participants are ACS Cognitive Services & ACS Recording, both of which are internal participants which are added by ACS when we enable call recording and transcription. There is only a single external participant - the "anonymous" caller.

One approach is to establish a valid connection with a known participant or number before attempting the transfer. This involves using the transfer API with a valid target participant who is not anonymous.

Not sure I followed this. When attempting the transfer, the target participant number is known. The error returned by ACS seems to indicate an issue with the transferee and not the target.

You may need to handle the anonymous caller differently, such as providing them with a known number or participant to transfer to.

Not sure I followed this either. Is the suggestion to provide the anonymous caller with the number and ask them to call into the target number themselves ? We're looking for an automated solution to be able to handle this.
Sampath 3,750 Reputation points Microsoft External Staff Moderator

2025-03-20T04:51:34.3433333+00:00
Hello @Sameer have you check this doc for Azure Communication Call Automation client library

For Incorrect caller ID displayed to the recipient

When you use direct routing, the caller ID information that is delivered to the call recipient is listed in the From and P-Asserted-Identity headers in the Session Initiation Protocol (SIP) options message.

The From header contains any of the following items:

The phone number that's used as an alternateCallerId property of a startCall method in Client Calling SDK. If an alternateCallerId wasn't provided, it's replaced with "anonymous".

The phone number string that's passed when creating a PhoneNumberIdentifier object in Call Automation SDK

The phone number of the original caller if an Call Automation SDK redirects the call.

The phone number selected as a Caller ID in Omnichannel Agent client application.

The P-Asserted-Identity header contains the phone number of the user who is billed for the call. The Privacy:id indicates that the information in the header has to be hidden from the call recipient.

Cause

If the information in the From and P-Asserted-Identity headers doesn't match, and if the Public Switched Telephone Network (PSTN) prioritizes the P-Asserted-Identity header information over the From header information, then incorrect information is displayed.

Resolution

To make sure that the correct caller ID is displayed to the call recipient, configure the SBC to either remove the P-Asserted-Identity header from the SIP INVITE message or modify its contents.
Sameer 40 Reputation points

2025-03-20T21:30:48.5666667+00:00

Hey @SnehaAgrawal-MSFT

To clarify & confirm, we obtained the phone number using the Azure Portal, so transferring folks that call in to that number isn't support by Azure at the moment ?
SnehaAgrawal-MSFT 22,706 Reputation points Moderator

2025-03-21T03:23:24.5433333+00:00

@Sameer Yes, As mentioned- Direct Offer which means phone numbers obtained through the Azure portal is work in progress but nothing definite and this point of time.

1 answer

SnehaAgrawal-MSFT 22,706 Reputation points Moderator

2025-03-20T17:42:21.3933333+00:00

@Sameer Apologies for Inconvenience here, I have reached to PG with your ask and as confirmed that ACS currently supports transferring anonymous callers in case of Direct Routing phone numbers, while Direct Offer (phone numbers obtained through the Azure portal) is work in progress but nothing definite and this point of time.

Further Feel free to express your feedback regarding the Azure Communication Services on Uservoice.

All input shared in these forums undergoes monitoring and review by the Microsoft engineering teams responsible for developing Azure.

Furthermore, users with similar requests have the opportunity to up-vote your post and contribute their comments.

Your feedback is actively considered in the ongoing development of the Azure.
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How to transfer a "anonymous" caller in Azure Communication Services ?

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