Following the merger of Tmobile and Orange in the UK (to Everything Everywhere), from 5th October 2010 you can jump networks when you don't have a signal. Really does maxmimise your network coverage by utilising both networks and it's a free upgrade.
Taken from Tmobile website:
Fancy talking and texting in more places than before
Now you can sign up to pick up a signal from Orange when you don't have a T-Mobile signal, so you can call and text in even more places.
Tmobile cusomters sign up here:https://www.t-mobile.co.uk/share/
Orange customers sign up herehttps://kareena.orange.co.uk/share/
Hope it's not been listed elsewhere - I did search!