I presume that you activated the SIM?
If so then the problem may be because you're using an iPhone.
Due to the stupid way that Apple do things iPhones need separate activation on certain networks.
This process sends a UK SMS behind the scenes.
On 48 this SMS will not send unless you have cash credit on your account to pay for the UK SMS (7.39c).
So you may need to top-up your cash credit balance (the minimum top-up is €1 as far as I recall) then switch off the phone, switch it on again and then wait a little while for it to automatically do this activation process.
Hope this helps.