It's not even simply when you move away. Case in point; My wife and eldest daughter are currently in Oz. With Three Mobile, they get free calls and SMS as though they were in the UK. My youngest daughter is still in the UK. My choices at the moment;
1) Turn off iMessage on my iPhone so that I can communicate with wife and daughter in Oz, but not be able to communicate with youngest daughter or anyone else using an iPhone in the UK.
2) Leave iMessage on and vice-versa.
3) Constantly switch between the two in the hope of intercepting one of the protocols.
It's a truly annoying high-jacking of a protocol by Apple.