Due to the stupid way that Apple do things iMessage needs to activate which uses a UK SMS and on 48 this cannot be sent unless you have enough cash credit on your account to pay for it (25c).
See here for example.
http://community.48months.ie/t5/Other-Help/iMessage-amp-FaceTime-Activation/td-p/798
http://community.48months.ie/t5/Other-Help/iMessage-activation/td-p/26167
http://community.48months.ie/t5/Other-Help/every-time-i-top-up-my-imessage-won-t-activate/td-p/18965
As I say, blame Apple for the stupid way that they do things which results in problems for iPhone users on certain networks when trying to get their phone working in the first place, roaming abroad, returning from roaming abroad, trying to use iMessage/FaceTime etc.
Hope this helps.
