When the phones are not on WiFi they are in effect on an external network. So they need to provide the IP address of your firewall and that firewall needs to be set up to allow traffic on port 5003 to pass through to the FMS box.
Thanks for your help. The client was recently struck by lightning and had to restart, repair, or replace most of their equipment. When the server consultant reconfigured their server, he failed to open the port for FileMaker Server. They're now back to normal.