Acrobits does let you set priority orders for codecs. And you can do so separately for 3G and WiFi connections.

Interestingly, though, when I set G.729 to top priority and G.711 to second priority for 3G, calls were always connected as G.711. When I disabled all codecs for 3G except G.729, calls were then connected as G.729. And, as I mentioned, in my tests thus far, have functioned fine.

rcrcr