[Serusers] ACK

Hello, I have two phones behind a Port Restricted Cone NAT (both in the same private area) and ser is running with another public IP. I want to call from one of those phone to the other. The call is set up and I can talk, but one Softphone shows me the message: "Waiting acknowledgement..."... and all followed SIP messages don't reach the other phone. I'm using a STUN server. Call from 14@xxx.xxx.xxx.xxx:5060 to 13@xxx.xxx.xxx.xxx:1024: 14 -> ser: ---------- IVITE 13@ip.of.ser.xxx@5060 (Contact: 14@192.168.1.101:5060) ser -> 13: ---------- INVITE 13@xxx.xxx.xxx.xxx:1024 (Contact: 14@xxx.xxx.xxx.xxx:5060) 13 -> ser --------- Trying and ringing (Contact: 13@xxx.xxx.xxx.xxx:5060) (!!!!!!!!!!! <-- wrong port !!!!!!!) 13 -> ser --------- OK (Contact: 13@xxx.xxx.xxx.xxx:5060) (!!!!!!!!!!! <-- wrong port !!!!!!!) ser -> 14 ---------- OK (Contact: 13@xxx.xxx.xxx.xxx:5060) (!!!!!!!!!!! <-- wrong port !!!!!!!) 14 -> ser ---------- ACK 13@xxx.xxx.xxx.xxx:5060 ser -> 14 --------- ACK 13@xxx.xxx.xxx.xxx:5060 14 -> ser ---------- ACK 13@xxx.xxx.xxx.xxx:5060 ... and so on... until timeout. Does anybody know what is the problem... or better: the solution? Thanks! Sebastian