Make sure of these things:
1. Profile in trasncoder is configure with proper G729 version (G729, g729a, g729b, g729ab)
2. Make sure Region between G711 and XCODER is configured for G711 and region between G729 zone and XCODER is G729
Region between G711 and G729 should be G729 for xcoder to kick in.
Example
IP Phone (g729) --- IVR (g711)
REGIONA REGIONB
TRANSCODER REGION C
Codec bw REGION A and B is G729
Codec bw REGION A and C is G729
Codec bw REGION B and C is G711
3. Device have access to XCODER via MRGL or NULL MRGL