Could be due to the stupid Apple iPhone activation process that requires an SMS message to be silently sent to a UK activation server and this will (also silently) fail on 48 if you don't have cash credit on your account to pay for the international text message.
I would suggest that you put €1 cash credit on your account and then turn the phone off and on again and see if it starts working after a while.
I'm assuming that the phone is already unlocked for use on any network.
If that doesn't help then you'll have to post more details including any error messages if applicable and/or contact 48 support for further assistance.
See the links in my signature below.
