Yes, this is annoying. However the fees are way less than calling from UK->country.

EG: when in the UK calling the US costs 56.2p minute, which is a complete ripoff as it should be cheaper than calling a UK mobile (virtually nil cost to do these calls). However, when actually in the US it's 15.6p. Same price differential for SMS.

Feel At Home basically means that your mobile will act the same as if you were in the UK.

Sending a text from the UK to a (for example) Spanish number incurs a cost outside of your allowance. So that same text would also incur a cost when you are in Spain.

I've set up Google Voice -> US SIP -> UK SIP -> 3 Mobile. This gives me a US number to give out to Americans (who don't want to dial my UK number) that they can text/call. I can then make calls when on Wifi or using a Phonecard app which automatically dials my UK SIP and forwards the call via the US SIP provider. I don't have to switch SIMs or give up my UK number.

This works well for me (since I travel between the US/UK often)

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?

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.

As I understood what I was told by their support, if you have a text or call allowance, that is used against local numbers too.