[Serusers] Asterisk SER problems

Hi All. I´m using Asterisk with SER. I can make call between two internal VoIP gateways or from na internal to external VoIP gateway. But when I get a external call, this call hang ups 5 seconds after and I reveive the following messages *CLI> -- Executing Dial("SIP/16008-3d17", "SIP/16007&SIP/16006|20|tr") in new stack -- Called 16007 -- Called 16006 -- SIP/16007-8c24 is ringing -- SIP/16007-8c24 answered SIP/16008-3d17 -- Attempting native bridge of SIP/16008-3d17 and SIP/16007-8c24 Mar 30 13:53:11 WARNING[1125685952]: chan_sip.c:495 retrans_pkt: Maximum retries exceeded on call 2eb06a983415436d4f2845a44dd9df5a(a)192.168.0.252 for seqno 102 (Request) Mar 30 13:53:12 WARNING[1125685952]: chan_sip.c:495 retrans_pkt: Maximum retries exceeded on call 2eb06a983415436d4f2845a44dd9df5a(a)192.168.0.252 for seqno 102 (Request) Mar 30 13:53:12 WARNING[1125685952]: chan_sip.c:495 retrans_pkt: Maximum retries exceeded on call D9A3-E113-71478237A5B7568-7@Octtel for seqno 8 (Response) == Spawn extension (sip, 1000, 1) exited non-zero on 'SIP/16008-3d17' Jadylson da Rocha Passos Bomfim Redevox Telecom Uberlandia +55 34 3234-7813 São Paulo +55 11 5055-6888 Móvel +55 34 9103-6854 --- Incoming mail is certified Virus Free. Checked by AVG anti-virus system (http://www.grisoft.com). Version: 6.0.643 / Virus Database: 411 - Release Date: 25/3/2004 --- Outgoing mail is certified Virus Free. Checked by AVG anti-virus system (http://www.grisoft.com). Version: 6.0.643 / Virus Database: 411 - Release Date: 25/3/2004