[Users] Problem Integrating OPENSER with ASTERISK

Hi there, I´m facing a problem with my voip callings. I use OPENSER as a proxy and an ASTERISK as the server which deals with all the SIP MESSAGES (including REGISTER) and is conected to PSTN. This solution works fine when an user tries to call to PSTN or any service on asterisk (i.e. VoiceMail), but if an user tries to call another user it´s not possible. It gives an error 500 - Server Internal Failure. Can anyone help me solve this problem ?? PS1 - Both servers are in the internet PS2 - I tried using both users with valid IP´s (not using nat) and the problem is the same. PS3 - I´m attaching 2 files, one with my openser configuration and another with debug of a call from 1234 to 4321 (See attached file: openser-debug.txt)(See attached file: openser.cfg.txt) Fídias Augusto