Text messages - UK iPhone adds US prefix 011 to texts

Hi all,

I updated my Iphone 4s last week to iOS 6.1 and while everything else works fine, I can't send texts anymore as all incoming texts are shown as 01144 xxx instead of +44 and nobody receives my text messages (guessing it adds 011 to my outgoing texts)

My first Iphone was a US AT&T 3GS, when I moved back to the UK, I got a new 4s and installed my backup (2 years ago). Now all of a sudden, it starts adding the 011 instead of + and nobody receives texts (except iMessages). I've tried changing the country/language settings back to UK, but didn't fix the issue. Any other thoughts other then a complete reset/reinstall?

Thank you,

Rob