You'll need to post a bit more info.
Are you trying to join 48 as a new customer?
Or are you already a customer and have suddenly lost service?
If you are just joining, have activated your SIM, have no service and have (as it seems?) an iPhone then you may need to put a small amount is cash credit on your 48 account to pay for the UK SMS (costing 25c) that Apple require to be sent by your phone (automatically) to activate the iPhone on a new network.
If this is the issue then blame Apple for the stupid way that they manage this process which is not required by any non iPhone phone.
If that doesn't help then post back with more info and/or contact 48 support for more assistance.
Hope this helps.