I did a Whatsapp call with a friend roaming in the UK (reported on earlier.) Sound quality was amazingly good, but I believe there were 5 second gaps here and there.

Latency was not particularly bad. I think that the latency might be less than one thinks because if you do a speedtest while roaming, based on GPS the app picks a local site (in the area in which you are roaming) to do the ping test. However, because you are on a VPN to the US, the ping goes from your phone, to the US, to the ping testing server in the roaming country, back to the US, then back to you in the roaming country over the VPN.

In contrast, the VOIP traffic might be smarter - so if you are talking via Whatsapp to someone in the US, the route might be from the roaming country to the US (via VPN), to Whatsapp US server, then from the Whatsapp US server back to the roaming country. That's two international hops for Whatsapp vs. 4 for the Speed and latency test.

I suppose it would be interesting to do a speed test from a roaming country where the testing server is in the US, not in the area of the roaming country (at least the app I used picks by GPS, not ip address location.)