I knew you'd be on Orange even before I opened the thread.
Got the same problem and have done since day 1. I can have full 3G+ connection and I still get texts saying I've missed a call even though the phone doesn't ring. Same when I dial out. Just keeps me hanging before failing.
Apparently it's the 3G masts which are over capacity, so the mast doesn't connect the call.
There are people on other networks having the same problem, as well as other phone users on Orange with the same issue.
My missus just got an HTC Wildfire on Orange and is having the same problems. She's not happy.