I suspect the voice might be handled differently than the data - i.e., it might not be routed through the US if you call from London to Manchester. However, with Tmobile at least (don't know about Sprint) the data traffic all goes by VPN to the US - you have a US IP address, and get US versions of websites.
This seems to be standard. I've used US, Irish, Italian and UK SIMs in the US and they all utilize VPNs to their home countries.
Originally Posted by
tehiota
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.