O2 with i phone worked for me in old college And afterwards but reception was a bit tricky in the chateau victoire. One tip no matter what phone you get is to get one of those Bluetooth headsets that people wear. Although you do have time in the evenings it's useful if you want to iron or bull your boots whilst having a lengthy conversation. Concurrent activity you see. Also it means you can leave your phone on the windowsill to make the most of the reception.
I found my iPhone on )2 had generally shite reception in most places, but given that it was turned off and left in my room when I was busy trotting round the place all day, it was not much of an issue.
On a mostly unrelated story (i.e. this didnt happen at sandhurst), I was once guard commander one sunday evening when a young soldier wandered into the guardroom with a bleeding cut on his head and said 'Corporal, I've been shot in the head!'
And so he had! He had been leaning out of his window to get phone reception when some drunken morons in the accomodation block opposite him decided to shoot him in the head with an air rifle! The poor bugger still had a pellet lodged in i think!
Why they where a) drunk on a sunday night (we used to have regimental PT monday morning, dark and early!) b) fcuking about with an air rifle in camp and c) thinking they could get away with it I do not know! They where in MOD Police cuffs in the guard room within the hour, one of em crying like a little girl.
So, get the best phone you can because leaning out of the window can be dangerous!
It's a general rule in the army that bases are placed in areas with poor or no mobile phone reception. The one exception I can think of was 1 Div HQ on Saif Sareea where they put up a mast in the middle of the desert, leading me to receive a phone call from my accountant in the middle of the GOC's evening conference call.
I think everyone worries about ending up in Nook Camp (and almost everyone does at some stage). Still, the burgers and steaks at the Angel down the road are still pretty good (don't bother with anything else on the menu as you'll still be hungry afterwards).