Our phone system has a public IP off our Verzion Fios. The firewall is configured to allow the public IPs of the home users. We have some VVX500's with the SIP server and SIP Transport server configured at the public IP on the NV7100. Is this possible? One phone seems to work, others do not and I am seeing SIP debug messages that contain the private IP address of the phone on the users home network.
You need to be sure SIP ALG is disabled on the home firewalls and your core firewall. Also turn on simple remote phone for those devices if your AOS version supports it. Also remember most home services are DHCP so there is a high probability that those IP Addresses may change and you will need to update firewall rules every time it happens.