Sounds like iMessage is trying to activate (again?) for some reason. This is one of the stupid things that Apple do to make life difficult for users. Instead of stuff like iMessage just working straight off they require this activation process first. This requires a UK sms to be sent to an activation server. On 48 international texts cost 25c each and won't send if you have no cash credit on your account. My suggestion is to top-up your cash credit balance by €1 (the minimum possible as far as I know) then reboot the phone and wait a while and see if iMessage finally activates. If it does you should also see that your cash credit balance has reduced to 75c.
Hope this helps.
I am not a 48 employee so if you need to contact 48 support then use option 3 or the chat facility here.