i think if the other end support SIP just make sur eyou are using the right SIP attributes like
non-secure SIP, SIP port
also CUCM for incoming support both TCP and UDP SIP
but for inbound it support one of then at a time so check the PBX what SIP it use TCP or UDP and change it to be compatible other than this it supposed to be standard, start your config with SIP trunk without MTP required Not checked
if dose not work try software MTP using MRGL assigned ot the SIP trunk
Are you still working on this? I am looking for a solution with the SV8500 PBX. I found on the NEC website that the 8500 is a SIP trunk certified solution, but i have still to find some type of integration guide.