iPhones are a pain in the arse due to the stupid way that Apple do things.
In this case try putting a small amount of cash credit in your account and then switch off and on again and then wait a while.
Or sync with iTunes.
Sometimes the issue here is that the iPhone has to activate (due to Apple's stupid requirements) and this process needs to send a UK SMS behind the scenes.
On 48 this international text costs 25c and if you don't have cash credit it never gets sent and the iPhone doesn't tell you what the problem is.
If that's not the problem then you'll probably have to contact 48 support for more advice.
Hope this helps.
