Is it an iPhone by any chance?
If so then welcome to the world of Apple's stupid iPhone activation policy.
iPhones (and iMessage and FaceTime etc.) have to activate on a new network.
Some are on Apple's "approved" list and this happens automatically.
Others, especially "virtual" networks like 48, are not and the process involves the phone silently sending an SMS message to a UK activation server.
On many networks (including 48) international SMS messages are not covered by the normal allowance bundles.
When the SMS fails to send Apple don't tell you this and you are left with this confusing message that you're getting.
To get around this you need to put a small amount of cash credit on your account to pay for international SMS messages and then turn the phone off and on again.
After a short time it should activate and start working normally.
God knows why Apple make their iPhone users put up with this nonsense.
Hope this helps.
