Some information from the CUCM SRND that may be useful to you:
* During normal operation, all voice, video, and web traffic from remote offices is sent across the IP WAN and terminates at Unified MPE in the central site.
* The same QoS and call admission control mechanisms used for the existing Cisco Unified Communications deployment are required. For purposes of call admission control, Unified MPE should be treated as a telephony gateway in the central site.
* In addition to G.711 mu-law and a-law codec support, Unified MPE also supports G.729a natively for optimal bandwidth usage across the IP WAN.
* Native G.729a codec support requires numerous Unified MPE system resources. A single G.729a audio call costs 5 SRUs from the overall system capacity pool. Deploying an external transcoding resource to transcode the voice stream from G.729a or other non-G.711 codec to G.711 codec would allow greater scalability with Unified MPE.
http://www.cisco.com/en/US/docs/voice_ip_comm/cucm/srnd/7x/mtplacex.html#wp1048966
I hope this helps.
Regards,
Michael.