I have an IPIPGW with H.323 to a Cisco Callmanager and a SIP dial-peer to a Service Provider. The Service Provider only accespts SIP Invite messages with SDP according to the SIP standard, so the "outbound Fast Start" with G.729 codec and the "require MTP" check box are configured on the CCM.
I have transcoder running on my IPIPGW/VGW. Calls coming from IP Phones in a G.711 Region can be routed through the SIP provider or the PSTN with no problems. Transcoding from G.711 to G.729 through the SIP provider is working fine.
However, if I have an IP Phone in a G.729 region, those phones are not being able to complete any calls to the PSTN or the SIP provider. Calls disconnect immediately once the called party rings.