Some good stuff here on MTP requirements with SIP Trunks..
Media Termination Point Required
“Enabling this option on the SIP trunk assigns an MTP for every outbound call. This option does not support codec pass-through mode, which imposes a single codec (G.711 or G.729) limitation over the SIP trunk, thus limiting media to voice calls only. With this option enabled, calls over the trunk uses MTPs assigned to the trunk rather than using calling device MTPs, which forces the media to follow the same signaling path”