I'm having this issue too.
Asterisk registering to a sip trunks on the wan.
after a while, no return register packets are received from the voip provider. this occurs almost twice per day.
If 2 trunks are registering, sometimes only one goes down.
Things we've tried on the asterisk box:
nat=yes
changing keepalives
What resolves the issue (on it's own):
server reboot
switch reboot
router reboot
What does not resolve the issue:
bringing the interface down/up on the server.
I think this may possibly be a nat issue to port 5060 any ideas anyone?