The networks work in a funny way, i know cos i'm a staff trainer for Orange

You have your masts but you also have something called an HLR (home location register)

Your home location register is located by the numbers on your sim card, all orange sim cards begin 894412 then you have three numbers could be 114, the last two numbers of this are your sims profile number (bit like a version number, the higher the number the better the sim storage) then another two numbers say 11 or something, these two numbers are you HLR number.

As you move around the country your phone automatically locks onto the nearest or strongest mast, however your HLR never moves, we have about 40 HLR's around the country, based in tannochside, liverpool and various other locations around the country

I live in Newcastle and my HLR might well be based in Tannochside, my dad is on Orange and he also lives in Newcastle but his HLR might be based in Liverpool, if i am standing next to my dad and my HLR goes down, my dad will still be able to make calls but i won't, although the phone may still show signal

If your phone goes down it may not always be mast related.