I've just done an online chat with O2 and the guy said that it's a known issue and I just to have wait until they fix it.... 24-48 hours and the only way I will know it's fixed is that I will have to keep turning my phone off and back on again, and testing afterwards to see if I can make a call or not ...!
I can receive calls, and send /receive texts. Most weird. It doesn't seem to be location-specific, as DH and I have exactly the same type of phone and his is working fine (and it's in the same room as me).
Anyone else? Or was the tech guy at O2 fobbing me off?!