How to fix a Blackberry Bold 9900 that says "unknown subscriber"?

Hi,

I've had this phone for a year and several months now and today I've had trouble texting, I keep receiving the message "Unknown subscriber" with a little red cross next to it. I also noticed today that my network has gone from 3G to 3g, however the little white blackberry logo is still showing in the right hand corner and my blackberry messenger still appears to be working.
I've tried hard and soft reboot, any suggestions?

Thanks