Voice is possible over LTE-M but it’s sort of the wrong technology for it. It’s designed for low power so the data rate is quite slow and the latency won’t work for two people trying to talk to each other.

It’s a similar speed to Bluetooth LE (not Bluetooth audio which headphones use). Sub 100kBps

NTN will be even slower. Few kBps. Really intended for sensor data or machine related actions