Thanks everyone. It sounds like I'm going to have to bite the bullet and ring Orange and hope they keep to what they say on the phone.
So just to show I'm not exaggerating about how bad O2 is for me, my fiance tried to call me just now.
1. He rings me, I pick up. The call is terminated, with the hilarious error message "Call Failure: Call Failed". I took a screenshot of it!
2. I ring him back, and he can't hear me. I end the call.
3. He cals me again, and he can hear me for about 15 seconds (just long enough to get the "omg we can actually hear each other, what a concept!" out of the way) before a really loud buzzing sound comes on the line and he can barely hear me at all, so I have to shout "KEEP TALKING I CAN HEAR YOU JUST TELL ME WHAT YOU WANTED TO SAY ARRRGGHH" and he spits out his message before it becomes unbearable again.
See why I want to switch?