With the advent of mobiles, most of the boxes are redundant. The original ones were flogged off for a fiver, I seem to remember, and are now available at £1000 each from various entrepreneurs.
In some places - notably the hills around the Conwy valley - you still need them. When our 'phones go off, there's no mobile signal to compensate, so we actually need a 'phone box to stand in for 45 minutes while we try to translate Urdu from the BT helpdesk into recognisable English. Joy.