When was it working ok last?
What happened in the meantime?
Was there a software update or anything?
Is the phone unlocked for use on any network?
Depending on the circumstances it could need (re)activation on the 48 network due to the stupid way that Apple does these things.
If that is the case (and there's no way too know for sure) then it will silently send a text message to a UK activation server.
And this will fail silently if you don't have an active membership plan and cash credit on your account to pay for the international text message.
So one thing to try might be to put €1 cash credit on your account, ensure that you have an active membership plan, then reboot the phone, wait a little while and see if that makes any difference.
If not then you'll probably need to contact 48 support for further assistance.
If you need to contact them then see the links in my signature below.
