Thanks vivek...we are not of internet so its bit problematic sharing the wireshark trace but the NEC voice card sends SIP 415 unsupported media type when cucm sends a sip invite (cisco phone dials the route pattern pointing to the SIP trunk).
When the NEC exchange dials the cisco phone number, we can see in wireshark capture that no request is coming from NEC exchange (the ip address of the NEC exchange is not seen at all)
By default, CUCM uses SIP delayed offer which is not liked by many vendors in initial call setup. Try to modify the SIP profile and enable SIP early offer. Assign this SIP profile to SIP trunk and reset the SIP trunk. See if there is any improvement.