The savings are potentially huge for both parties, Orange and T-Mobile share probably thousands of masts, I believe the 'owner' of the mast being shared charges for the privelige, so the rationalisation of cell sites is extremely beneficial.
However, to handle the increase in the number of users using one big network, I guesss each site's capacity is going to have to be looked at, so maybe (for example) leaving a T-Mobile antennae on an Orange mast is the easiest solution?
Any cellular comms gurus in the house?