asterisk users - Jan 2005 - incoming calls timing out.

I have an stanaphone number for incoming calls, I am using asterisk and it
works fine at first, but after a few hours the incoming calls fail they go
directly to the stanaphone mailbox.

I believe the registration probably times out, or maybe it is the dynamic
IP changing.

Is there a way to fix this problem?
Any help will be greatly apreciated.

Thanks

Sergio