[Serusers] problems when going to pstn

Hi all, I'm writting to ask for suggestion. I'm able to communicate sip-to-sip using SER. But when one user wants to talk with somebody in the pstn, I receive two different errors, depending on the gateway. Look ... This first message came from a Mera sip server. U IP_MERA:5060 -> IP_SER:5060 SIP/2.0 415 Unsupported Media Type. v: SIP/2.0/UDP IP_SER:5060;branch=z9hG4bKad03.0a8597a1.0. v: SIP/2.0/UDP IP_UA:15172;branch=z9hG4bK2413941036. Record-Route: <sip:IP_MERA:5060;maddr=38.117.22.34>. Record-Route: <sip:IP_SER;ftag=2859935538;lr=on>. f: "1991117" <sip:1991117@IP_SER>;tag=2859935538. t: <sip:011543515684778@IP_SER>;tag=2eff4b002937ff10ff0000045a5b1432. i: 6ad7a895fbb61ed09f5b2ea36b6612ce(a)192.168.1.30. CSeq: 1 INVITE. P-Session-Timeout: 7200. Server: MERA MSIP v.1.0.1. l: 0. This one from a cisco AS5350 U IP_CISCO:5060 -> IP_SER:5060 SIP/2.0 400 Bad Request - 'Invalid SDP information'. v: SIP/2.0/UDP IP_SER;branch=z9hG4bK2817.1aa690e7.0. v: SIP/2.0/UDP IP_UA:15188;branch=z9hG4bK3914033240. f: "1991117" <sip:1991117@IP_SER>;tag=2962826014. t: <sip:3055383346@IP_SER>. i: 332ee2e17a14427caeb2feaf2da22a3a(a)192.168.1.30. CSeq: 2 INVITE. P-Session-Timeout: 7200. l: 0. I do not get it. Why could I communicate sip-to-sip just fine, but, when going to pstn, I receive this errors. And, moreover, they are different. I'm only having this error with this particular user, which is behind a router doing NAT. Do you have any ideas of what could I do to solve this ... ? Or what could be the problem ? Best regards, Lucas -- No virus found in this outgoing message. Checked by AVG Anti-Virus. Version: 7.0.323 / Virus Database: 267.7.11/26 - Release Date: 22/06/2005