I’m guessing that a ‘poisoned’ NAT association occurs during the network outage and the frequent retries keeps it alive. During the reboot, it times out. Try setting both General Retry Interval and Qualify Frequency for the trunk to 600.
If that doesn’t help, at the Asterisk console, type
pjsip set logger on
and cause the failure. If you see proper REGISTER requests going out and nothing coming back, use tcpdump to capture traffic to see whether it’s a firewall issue (the capture is before the firewall for inbound packets and after the firewall for outbound).
If neither pjsip nor the firewall are malfunctioning, please post:
ISP? Modem make/model? Separate router/firewall (if any) make/model/version? Any complex network config (multiple NICs, VLAN, VPN, etc.)?
If you have a static public IP address, you can make the system more robust by not using registration. I’m not a VoIP.ms customer, but believe that you can send incoming calls to a SIP URI, and that outgoing calls do not require registration. Also, for better security, set up the account for IP authentication.