asterisk users - Apr 2006 - SIP client looses register and then i need to restart my pc to get registered on Asterisk 1.2.5

Hi all,

I've a some users on my network, reporting this:

Sjphone is registered , and some times just looses registry in
Asterisk, I don't know if it is expiration ( instead of loosing
registry).

Then to get registered again they need to restart their own PC.

Why could this beeing happening?

Best regards,
Marco Mouta