Is the other phone unlocked for use on any network?
What is the verbatim message that you're getting and where are you getting this - e.g. on the phone itself or when you log into your My48 account?
What make and model of phone is it?
If it's an iPhone then the silly Apple/iPhone activation process may need cash credit on your account in order to pay for the activation text message that the iPhone will silently try to send to a UK activation server and this will fail if you don't have cash credit on your account to pay for the international text.
If you need to contact 48 support about this then see the links in my signature below.
