Well, I have used VoIP, Skype, landline, probably some other stuff that I don't remember.... and I must say, there is nothing clearly better than 2g voice. I have used Skype, Discord, Teamspeak with my friends, and they had really expensive and good microphones, but there wasn't any big difference between that and our standard cellphone operators. Yeah, there was a difference, but it was only marginal. And I'm an audiophile, so I really notice these things. But the thing is, it depends on what phone and what operator's service you use. Some phones have better microphones than others, the same ca be said about the speakers and antennas. These things bottleneck the quality most often. And they have improved quite a lot lately. Using recent smartphones, the voice quality is hard to distinguish from high bitrate VoIP calls (which is supposed to be better than VoLTE) using expensive microphones and headphones. On the other hand, using an old feature phone to make calls makes it hardly any better than landline phones. I guess you just need a very good 2g network and good microphones/speakers on the phones of both participants of the conversation. In that case, you shouldn't really feel any difference between 2g and 4g calls.