Once off Apple iPhone, iMessage and FaceTime activation can require a UK SMS to be sent which on 48 will require cash credit (25c per international SMS).
Not sure why you're getting the message all of the sudden but that's the reason why it can appear in some situations.
See here for example:
https://support.apple.com/en-ie/HT201422When trying to contact that person do you know if iMessage is trying to send SMS or iMessage "native" messages?
Is the other person an iPhone user?
Is this of any use?
https://discussions.apple.com/thread/5761230?tstart=0If not then maybe try browsing/searching the other Apple forums and help sites for info.
If all else fails then you may need to contact 48 support for assistance:
http://help.48months.ie/requests/newHope this helps.