We have a CCM 4.3 with a sip trunk going to our provider. The traffic is passing our firewall where it is being natted.
This configuration is working fine.
We are now planning to migrate to CUCM 6.1. When placing an inbound call via the SIP trunk to the CUCM 6.1 , the caller keeps getting ring tone even when the ipphone has picked up the call.
Checking the FW logs reveals that the voice packet is being dropped because of an illegal sip redirect from the CUCM to the IP of the ipphone.
So I'm guessing the MTP is not being used in the SIP trunk for some reason.