By POP, I'm referring to the Carrier's POP. In my voip.ms example, they have pops around the world, but the same would/should be true in the cae of T-Mobile. Even with a US #, if T-Mobile has a POP in London (and I was in London), I'd want to choose that to get on T-Mobile's network ASAP since that's within their control. T-Mobile would then backhaul it to their voice gateway.

I must have missed where they're VPN'ing the traffic back to the US. I would have assumed (maybe incorrectly) that they'd use encrypted (SSL, etc) to a geographically load balanced pop to get voice off the internet ASAP.