New IP address cannot register until I add the IP to Firewall "Networks" as trusted

Each time my CEO's IP address changes at home he is unable to register with the PBX until I add his IP to the firewall under Networks, setting it to either Trusted (excluded from firewall) or Local (Local trusted traffic). He also travels often using a softphone. Often I need to whitelist IP’s of whatever wifi network he’s on. Is there a solution for this? It’s my understanding that the PBX should clear any IP that successfully registers with the correct credentials. However this is not happening. 

Thanks