I found the problem. The phone uses RPORT and switches to the port reported by FS instead of using the one specified in the provisioning. :(

On Wed, Apr 14, 2010 at 8:45 AM, Jonas Gauffin <jonas.gauffin@gmail.com> wrote:
Hello,
 
I got a problem with Snom phones where they report a different port than the one provisioned. I tried to talk with the snom support without success.
 
REGISTER sip:phone.gateon.se:5070 SIP/2.0
Via: SIP/2.0/UDP 217.31.xxx.xxx:26062;branch=z9hG4bK-u1y0v7is9ix0;rport
From: "Helene XXX" <sip:u1000028@phone.xxxx.se:5070>;tag=kjaa26j1qv
To: "Helene xxx" <sip:u1000028@phone.xxxx.se:5070>
Call-ID: 3c267019ee2d-ni5vq3v88za6
CSeq: 133 REGISTER
Max-Forwards: 70
Contact: <sip:u1000028@217.31.xxx.xxx:26062;line=yhkplirb>;reg-id=1;q=1.0;+sip.instance="<urn:uuid:1f2c528a-18f1-435d-ad61-2be3d1a88425>";audio;mobility="fixed";duplex="full";description="snom360";actor="principal";events="dialog";methods="INVITE,ACK,CANCEL,BYE,REFER,OPTIONS,NOTIFY,SUBSCRIBE,PRACK,MESSAGE,INFO"
User-Agent: snom360/7.3.30
Supported: gruu
Allow-Events: dialog
X-Real-IP: 192.168.3.109
Expires: 3600
Content-Length: 0
The port that I provisioned (and I've doublechecked in the webinterface) is 5071 and not 26062.
 
Have anyone else got this problem and solved it?
 
Regards,
  Jonas