IP Office does not provide in-band ringback to incoming SIP trunk calls. The only normal scenario in which an incoming SIP trunk call hears in-band ringback occurs when the call terminates on an analog trunk. With analog trunks, the media path is cut through immediately because IP Office has no way of determining the state (ringing, busy, answered) of the trunk.
IP Office can connect “early” media before the call is answered by sending a 183 Session Progress response. This is only done when the following two conditions are met:
A PROGRESS (in-band tone indication or 183 Session Progress with SDP) message is received from the destination. This can only happen in a SIP-to-PRI or SIP-to-SIP tandem call scenario.
The INVITE message contains SDP.
IP Office does not attempt to connect early media on PROGRESS when there is no SDP in the initial INVITE, since this is unlikely to succeed. The likely reason there is no SDP in the INVITE is probably that the originating system does not know the originator’s media address yet. A typical scenario where this is the case occurs when the call on the originating system comes from an H.323 SlowStart trunk.
