nano3g ip.access femtocells and transcoding

Dear Osmocom community, Did somebody try to use nano3g ip.access femtocells and Osmocom system with external call handling? What I am trying to do is to make calls work between VoIP phones connected to PBX and UEs connected to Osmocom system. I've tried two PBXs: Kamailio with rtpengine and Asterisk. Signalling worked good between VoIP phone and UE with both PBXs; calls between two UEs registered on Osmocom system worked good also... But I fail to do transcoding (calls between VoIP phone and UE). The problem is that it is unknown which codec is used by nano3g ip.access femtocell. So, the question is: Do you know what codecs were used for speech in your setup with nano3g ip.access femtocells? Or do you know how to find out that information? (It was stated by ip.access that they use AMR but I failed to match AMR specs with RTP payload that was sent by femtocell...). I would be very grateful for any ideas. Kind regards, Mykola