There's also vulnerabilities to using your cell carrier's 3G voice service. More or less than VoLTE or VoWIFI? Who is to know?
Note: VoLTE and VoWIFI are basically the same thing, differing in whose network the data is routed over. VoLTE the call is routed over cell network, VoWIFI the call is routed over whatever Wifi AP you happen to be connected to. If I recall correctly, its fundamentally SIP, like any VoIP PSTN phone, and having media layer encryption (which some other VoIP phone may or may not have). Copper pair phone lines are basically the only alternative to SIP nowadays for access to PSTN, and I don't know if anyone is even connecting those anymore.
If you want to break away from the PSTN vulnerabilities, you'll need to use some other means of voice[/video] communication.