I've sort of suspected this the case for a while. On VPN, MMS and Visual Voicemail still work on Android. Both of these require direct mobile access or they will get rejected (sometimes they are only on the mobile network, or else they requests get rejected if they don't come from within the mobile network). I suspect the same is true of VoLTE. If there is a VPN, that would mess things up.

I found this out since on Mobile Linux, if you enable VPN, the VPN breaks all of those.

I don't think there is a clear way to fix this on Android without breaking a lot of expected functionalty.