This Best Practice details the local transfer feature limitation when SBC Edgepasses through an RTP stream in Proxy Mode.
When the Sonus SBC Edgecompletes a local transfer, the transferee party receives the stream with different RTP header parameter(s) (such as SSRC, timestamp, etc.). This is because the original end point (transferor) is replaced by a new endpoint due to the transfer. Since the RTP stream is passed through by the FPP, there is no re-packetization done for any of the RTP header parameters, and the stream is passed end-to-end transparently.
As a result, this might cause media issues (i.e., no audio) in the end point, since the endpoint may not be equipped to handle changes in the RTP parameters and would discard the RTP stream, and may disconnect the call. This would be the case, even after processing the Re-invite from the SBC due to the transfer.
To handle the transfer in RTP Proxy mode reliably, the SBC rejects the REFER and raises an event in local register mode.
REFER pass-through should be enabled by configuring the SBC (see Creating and Modifying SIP Signaling Groups), which forces the transferee to handle the REFER directly.