You seem to be using an iPhone?
If that's the case then it's possible that the problem is that iMessage needs to be activated?
Due to the stupid way that Apple do things with iPhones this may involve the phone silently sending a UK SMS which costs 25c (at the moment - c. 8c from May) to send charged to your cash credit balance.
If you don't have the cash credit to cover this then it will fail but Apple/iPhone won't give you any indication of the problem.
It's a really dumb setup but that's Apple for you.
Try putting a small amount of cash credit on your account - e.g. €1 - then see if messages eventually start sending ok.
Maybe power off and on again in case that helps.
And if you ever ditch iPhones and use another phone then you'll probably have to remember to deregister from iMessage otherwise you probably won't get messages sent to you by others. Again blame Apple for this dumb setup that just makes things awkward for customers.
If that doesn't help then you may need to contact 48 support for more advice/assistance.
Hope this helps.