On LTE and 5G, phone calls are already just VoIP (VoLTE/VoNR) over a pure data network, ideally you'd decouple your VoIP service completely from the dumb data pipe.

Of course the last thing that any ISP (wired or wireless) wants to do is commoditize their service as a a dumb data pipe - they all want the margins of those over-the-top services.