A big clarification, an MTP cannot work as an XCODER. An XCODER can work as an MTP. The MTP cannot be changing from G711 to G729 magically. Specially when you're using SW MTP on the server. Any XCODER are external DSPs to the server.
When MTP is running in software on Cisco Unified Communications Manager, the resource supports 48 MTP sessions. When MTP is running on a separate Cisco Unified Communications Manager appliance (server), the resource supports up to 128 MTP sessions. In addition, Cisco Voice Gateway Routers also can provide MTP services.
http://www.cisco.com/en/US/docs/voice_ip_comm/cucm/admin/7_0_1/ccmsys/a05dsp.html
To understand exactly what an MTP can do read this
Media Termination Point (MTP)
http://www.cisco.com/en/US/docs/voice_ip_comm/cucm/srnd/7x/media.html#wp1046314
Most phones use G.729a and G.729ab. If you have a model in mind just search the model on CCO and look at the datasheet.
HTH
java
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HTH
java
if this helps, please rate