"Uncompressed" voice "needs" 64kbps (hence ISDN was 2 voice channels etc). If you use a modern codec, you could probably get away with less, for high-quality mono voice. Note that you might not want to use VBR, for other reasons (I thought this was already submitted to hn - couldn't find it - so here's a fresh submission):

"Attacks on packet length may be surprisingly good: Hookt on fon-iks": https://news.ycombinator.com/item?id=10462363

That said, I suspect that eg: Ogg Vorbis at 64kbs CBR would be fine for voice, assuming you could keep the latency down.

Latency and drops would probably be the biggest issue. And you can of course prove, that latency sets a limit to the number of hops -- say if you'd want to hop through a European jurisdiction from the West Coast.

I'm actually planning to (try to) move to a pure data-oriented (not sure if there will remain a cellular part, or if I'll just use wlan) (probably) SIP for all my phone needs. That'll probably involve a SIP server on dedicated (rented) hw in Germany, and VPN from the phone/laptop. Not quite sure yet if it'll work out, or if it'll be good enough.