3CX supports G.729 Annex A codec. If you do not have voice while using G.729 it (most probably) means that your PBX has received non-stangard codec name in SDP. We can not foresee all possible deviations, so we support only standard (registered by IANA) name for G.729 codec. Re "3CX codes" - what exactly do you mean? Dialing codes or what?