I'm curious why DuckDNS is picking up on the VPN IP instead of the local one? no split tunneling?
Any possibility that the home user has a router that supports DynDNS? If so, set that up should solve the problem.
Good call on the split tunneling. Checking...
If it doesn't split, then the phone traffic is likely going down the tunnel, too. Or else you are checking from a different location than the phone.
Not likely, if its a full tunnel the external ip WILL always be the IP of where the tunnels ends with everything flowing through the tunnel.