Possibly an outgoing call bar either set in the phones menu or by O2 for some reason.
Call CS to make sure your account is paid up and not in dispute for anything.
All contact details here if you can't dial short codes either from your phone
http://www.o2.co.uk/contactus
I have no affiliation whatsoever with O2 or any subsidiary companies. Comments posted are entirely of my own opinion. This is not Customer Service so we are unable to help with account specific issues.
Please select the post that helped you best and mark as the solution. This helps other members in resolving their issues faster. Thank you.