[Freeswitch-users] Attended transfer - no audio

Hi, we have a Freeswitch server on a public IP and a few phones behind NAT. The phones are configured to use STUN and can register and call each other fine. The problem is that after attended transfer (using the mechanism the phones provide - REFER) is finished, the two parties can't hear each other. This problem doesn't occur when the phones are in the same subnet as Freeswitch. I know this isn't enough information to solve the problem, but do you have any hints on how to debug this? Are there any specific Freeswitch settings that could help us? Thanks, Jan