A lot of good things are said already. VoWIFI goes by IPsec tunnel to the telecom network and many ISPs will block IPs that aren't from the home country of the ISP. Also the latency may even be part of the reason it does not work.
IPsec tunnel does not want to play nicely with many VPN scenarios, so it may indeed not work then. Excluding those devices will help then.
ISP will block based on IP because they want to prevent you bypassing roaming using VoWIFI. You wouldn't want your customer NOT to pay roaming fees, right?
I did get it to work fine with a OpenVPN TAP tunnel to my home connection, but that is not ideal for many many reasons.