asterisk users - Apr 2007 - no reply to our critical packet

Hello
My asterisk is receiving calls from OpenSER but all calls hangup in 20 
seconds.
This only happens because Im using Asterisk2Billing's AGI (without 
A2Billing it doesnt hang up).
does someone knows whats the problem??

Here is my Asterisk debug:
(xxx.xxx.xxx.xxx  -> the phone's IP)



Apr 10 02:03:02 WARNING[6996]: res_musiconhold.c:508 monmp3thread: 
Unable to spawn mp3player
Apr 10 02:04:18 NOTICE[8349]: rtp.c:331 process_rfc3389: Comfort noise 
support incomplete in Asterisk (RFC 3389). Please turn off on client if 
possible. Client IP: xxx.xxx.xxx.xxx
Apr 10 02:04:19 WARNING[7007]: chan_sip.c:1228 retrans_pkt: Maximum 
retries exceeded on transmission 
CAB5E4C9-8B95-42D4-9A93-FB996EDC43A2@xxx.xxx.xxx.xxx for seqno 12282 
(Critical Response)
Apr 10 02:04:19 WARNING[7007]: chan_sip.c:1245 retrans_pkt: Hanging up 
call CAB5E4C9-8B95-42D4-9A93-FB996EDC43A2@xxx.xxx.xxx.xxx - no reply to 
our critical packet.
Apr 10 02:06:01 NOTICE[8360]: rtp.c:331 process_rfc3389: Comfort noise 
support incomplete in Asterisk (RFC 3389). Please turn off on client if 
possible. Client IP: xxx.xxx.xxx.xxx


Thanks for the help
Regards
Joao Pereira