The following summary gives caveats that apply to MTP transcoding:
Make sure that each Cisco CallManager has its own MTP transcoding resource configured.
If transcoding is required between Cisco CallManager clusters, make sure that the intercluster trunk is configured with an MTP resource. All calls between Cisco CallManager clusters will go through the MTPs.
If all n MTP transcoding sessions are utilized, and an n + 1 connection is attempted, the next call will complete without using the MTP transcoding resource. If this call attempted to use the software MTP function to provide supplementary services, the call would connect, but any attempt to use supplementary services would fail and could result in call disconnection. If the call attempted to use the transcoding features, the call would connect directly, but no audio would be received. If a transcoder is required but not available, the call would not connect.
Try:
http://www.cisco.com/en/US/products/sw/voicesw/ps556/products_administration_guide_chapter09186a00801ec5bb.html