I have created a SIP trunk from CUCM ver 7.1 towards a Nortel PBX ver 5.0.
The scenario is like:
Phone A----->CUCM 7.1-----SIP Trunk--->Nortel CS 5.0-------> Phone B
The regions are configured to be in G.729. As per the interop document we have to check MTP required check box so as to support the SIP Early Media Offer.
I have configured a transcoder, to have the MTP requirement. I have ensured that transcoding is invoked in all the calls.
When calls are made from Phone A connected to CUCM to Phone B connected to Nortel, some supplementary features are not working.
If Phone A calls Phone B, and Phone A presses HOLD and RESUME, it works fine at the Cisco end. But at the same time, when Phone B presses HOLD and RESUME, Phone B is able to hear Phone A but Phone A is not able to hear Phone B.
The strange thing is , after the previuos HOLD retreival failure for Phone B, If I press HOLD and RESUME at the Phone A the voice path is re-established.
Any idea of this strange behaviour?