Surely, that is the answer. If a 'dummy' cell tower is set up inside the building (just enough power to 'attract' all phones inside the walls). It can simply refuse to give a connection (it wouldn't have the connections to the networks anyway). I presume that if you are nearer a normal working cell but all its lines are busy, you get a busy signal - even if you are within range of another (weaker) cell that does have lines?

The building walls would provide sufficient attenuation at 2Ghz. Windows and doors might need the foil or mesh treatment though...