[Serusers] ACK

Hi! I noticed a "phenomenon" that I don't understand: I make accounting with ser. If all works fine, ser puts into the database: sip_method: ACK i_uri: sip:0182011542614xxxxxx@xxx.xxx.xxx.xxx:5060 (xxx.xxx.xxx.xxx = IP of PSTN-Gateway) ... and after the call, the BYE message reaches my server. But sometimes, the ACK is logged in the database like this (in the most cases form users "far away"): sip_method: ACK i_uri: sip:0182011542614xxxxxx@my-ip:5060 In this case, the BYE message doesn't reach my server. Does anybody know what could be the reason and why ser logs the ACK from himself? Many thanks! Sebastian