I don't know the VoLTE protocols, but for regular PBX-style VOIP, the voice compression is good enough and the voice payload in the packets is small enough that most of the bandwidth is used for IP/UDP/RDP headers, not the actual voice. There are way too many standards to choose from, but most of them run about 5KB/sec or less (that's bytes, not bits), so about 300 KB/min, or about 3000 min for 1 GB. There are people who use that much voice time, but not many :-) I'd expect that for a while you'll see multiple different standards for handling hd-mobile-to-hd-mobile, sd-mobile-to-sd-mobile, mobile-to-wireline, mobile-to-other-mobile-carrier, etc.
Back around 1990, I went to a technology talk by a guy from MCI who thought that the conflicting economics of offering voice and video on the same network were going to be a serious problem for telcos - video at the time meant ~1.5-3 Mbps corporate teleconferencing, and either you could price video too high to sell much of it, or you could sell T1 bandwidth cheaply enough to make videoconferencing affordable, in which case you'd undercut your voice pricing because companies would buy your video T1s to interconnect their PBXs for cheap. Better video compression got us out of that hole for a few years (384kbps or especially 128kbps video didn't cause that much trouble), but the Internet came along and started doing the same technological undercutting, VOIP started becoming feasible, etc. Mobile phones gave us a way to charge lots of money per minute again, but Moore's Law is still relentless.
Disclaimer: I do work for AT&T, but I do computer security, not mobile phones, so I have no idea what they're planning to charge for this, this is my own opinion, not the company's, blah blah blah. On the other hand, I have been doing various kinds of telco things for many generations of technology :-)