[Serusers] SIPSAK - wrong behaviour??

Hi ALL! I am trying to run some tests under sipsak but I obtain strange result I invoke sip stack with the following command: sipsak -IU -vv -s sip:sipsak@scscf.ims.touk.pl -a sipsak -l 5061 -H 192.168.0.129 The registration proces runs without any problem, user registers and then sipsak sends invite to ser. And here comes the strange thing 1. sipsak sends invite to ser: 2. Ser sends the proxy authentication required response 407 (from ngrep) 3. sipsak receives the message andresponses with the ACK???? Ive placed the ngrep from the sipsak socket in the attached file. Please help mewith this one! Best Tomasz