Just made my first voice call on 4G (LTE AT&T Network) using Purism awesim on an L5 with US modem. I also received a call and while the ringing is laggy, the call quality is excellent and i and the other person heard the ringing. Plus it did not drop to 3G neither has the modem or connection dropped out at all last hour testing it.
It took a while to get the correct awesim activated since my account had tons of dupes for some reason but with help from purism support they worked it out, couldnt be happier.
Honestly i have no idea what would be different technically between 3G voice and 4G voice, i assume both use a digital signal and connection not analogue, and wouldnt that by definition make it VOIP wouldnt that even include 1G, 2G?