[koyote]

Can't you just use Google Voice on Three's 4g connection/Wifi to make/receive calls/texts on your Google Voice number using Hangouts?

What's the purpose of the two SIPs?

[geocar]

Three doesn't have any 4G when roaming, and the 4G isn't that great even in London (compared to say: Vodafone).

Texting with Google Voice is fine though.

The two sips are registered to the same installation of asterisk: The UK provider gives me a UK number and rates equivalent to a UK landline.

The US provider gives me a US number (which is registered with google voice) and rates equivalent to a US landline.