asterisk users - Feb 2005 - Problems with SIP Registration at PSTN Provider

Hi together,

I have a asterisk running on a Debian testing system running 
"flawlessly" at least after starting the asterisk.
The Server its running on has a fixed IP, no NAT, whatsoever and is 
reachable all the time. The Firewall has holes on port 5060 and for the 
RTP-range that asterisk is configured on.

In my sip.conf I have a few register=>lines and I can receive calls over 
those accounts.
However after a few hours, days whatsoever asterisk is still thinking 
that those registrations are valid(CLI) but my provider (sipgate.de) 
sees the asterisk as offline and hence does not forward calls to my 
asterisk box. I tried playing with the *expirey values in the sip.conf 
but I have no clue how to test it properly cause it can work fine for 
days and then stop working ...
Sipgate is running SER btw.

Is that a known problem  ? Has anyone a solution other than restarting 
asterisk every hour or so ?

regards,
    Magnus Jungsbluth