We're trying to setup an environment where a SIP attendant (or controlled) transfer, not blind, is allowed.
In detail, any incoming call comes through a voice gateway and goes to a SIP IVR. When IVR must transfer the call to an agent, it makes a second call to the destination agent (through a SIP INVITE) connected to an SCCP phone on UCCM 6.1 and, if the agent answers the call, it completes the transfer sending a SIP REFER to the original caller with the header replace (containing the call-id of the agent call).
This scenario works fine as described (when the call comes through the voice gateway) but it doesn't work if the original call comes from another UCCM phone.
In this case, when the REFER is issued, we always get a 403 Forbidden error message. Attached you'll find the SIP trace collected from IVR with the list of all messages.
Could someone tell me if the SIP REFER/REPLACES method is supported by UCCM and, if allowed, where I can configure this?