This is probably the most common issue we encounter. There are a few things which can cause a device to lose registration, but most commonly. the issue is caused by NAT translations going stale on the customers router/firewall.

If rebooting the phone does not resolve the issue, the easiest way to resolve this is to reboot the router/firewall which is responsible for the NAT on your local network. This will clear the translation tables and get the phones working again.