Vi riporto la modalità descritta sul sito Three UK:

To activate the pass, firstly make sure data roaming is enabled. You’ll then also need to turn off your push notifications (they’re the things that tell you stuff like when you’ve got an email, when it’s your turn in a game or when someone’s tagged a pic of you). They need turning off to avoid out of bundle charges while you’re setting up the pass. How to do it differs from phone to phone, so make sure you’ve worked out how to sort it before you go away. Then just click on the link in the SMS. Follow the simple steps to buy the pass and then you can go wild online until midnight that day (UK time).