Earlier this year T-Mobile announced that they were merging with Orange. This didn’t matter too much to most at the time but its changed a little now. For a while they’ve been testing network sharing and its now live for customers to use. If you’re on T-Mobile you need to first text ‘Yes’ to 2121. This will switch your account to use both networks and whichever has a signal. You’ll get a text back saying that your account has been changed. Remember that at present if you’re a T-Mobile customer using the Orange network you only get 2G, not 3G so things like YouTube won’t work (unless you’re jailbroken and forcing it to).
I’ve tested it myself and it works flawlessly. If you’re an iPhone user you will need to make a few change to your settings. Goto Settings.app, click ‘General’ then ‘Network’ and turn on Data Roaming. If you want to force your phone into using Orange, click ‘Carrier’ and change to Orange (or T-Mobile Orange if its called that for you).