I recently used T-Mobile's 2G international roaming and primarily made WhatsApp audio calls - like you, I found the quality to be good and latency did not appear to be a problem at all. I did have the "low data usage" option for Calls selected.

I made a few FaceTime Audio calls too and they were just fine as well. Unfortunately, I forgot to try Google Hangouts and Bria with VoIP.ms (or other providers); I imagine they'd be fine.