Both Sprint and Tmobile now offer some variety of free international roaming with slowed data - 128 kbps for Tmobile, 64 kbps for Sprint.

I have seen various comparisons online of the data usage of various VOIP apps (including Whatsapp which allows no PSTN calls). However, those comparisons always show data usage on fairly high speed lines - which does not tell us who well, or if at all, the app can scale down to slower speeds in the case of a slow connection (such as the 2G speeds given for free by Sprint and Tmobile.)

Has anyone seen any comparisons of low bandwidth VOIP app usage and quality?

(And yes, I realize that there is the separate issue of latency given that it seems that the data goes over a VPN to the US.)