Yes - Why are you suspecting that it does not work? I have spent some time to go over this again.
Attached is a capture taken using a Broadvox account with only G729 enabled on the Advanced parameters.
We are using the same version 9. (you simply have to trust me

)
You see invite is sent with PCMU and G729 to PBX. 11.717
Since provider is bound to media server, Pbx sends all supported codecs to phone. 12.039
I set phone to only answer in G729 to avoid confusion - however the internal components can speak as they like because you do not need to save bandwidth on the internal lan. Media server prefers PCMU for internal communications - + quality, - effort to transcode to lower G729 area.. 14.979
As you can see RTP stream is sent in G729 to the broadvox media server. 14.997
200 ok with SDP in G729 is sent to broadvox sip server 15.098
Return media from Broadvox Media server is sent in G729 15.353
I have shown you how this works in Version 9.
Outbound calls work in the same way too.
Can you show me how in your case this does not work?
Let me know