Unable to transfer call from Teams Phone

Question

When transfer a call from SIP Trunk to Teams Phone, then transfer to PSTN, the transfer is failed. Under the sniffer capture, there is no RTP "Media" Packet sending from Teams (52.113.97.233:51744) to our SBC (45.125.8.6:7310):

Issue: Teams Phone Missing Media Packet

Issue Date: 16-Nov-2023

Issue Time: 11:19 (GMT +8 UTC)

Caller: 21259872

Callee:37429701

Answer 1

Hi @Raymond Lui

All transfers that use an SIP Refer message must go through the Microsoft Teams infrastructure. When the Microsoft SIP proxy sends an SIP Refer message to SBC, an SIP Invite message should be returned to the SIP proxy, not to PSTN or to any other destination. It is true even if the call is transferred to an external PSTN number. SBC doesn't have to parse the SIP Refer message to look for the transfer target. SBC should send the SIP Invite message together with the Request-URI (RURI) setting only to contents of the Refer-To header. It also should include the Referred-By header from the SIP Refer message. Make sure that the strings of the SIP Invite message are not changed, and that they are sent as the exact same strings that are provided in the SIP Refer message (especially in the Referred-By header). This is because these strings are used to identify calls, targets, and other important parts of a call transfer.

This official document mentions common issues about call transfer failure for your reference.

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