The reason that they work inside buildings is that microwave signals will come through the window openings and then reflect around inside the building. The reason it works so well today compared to the old analog cell signal is that the new technologies (CDMA) actually take advantage of the multipath signals (reflected), this is also why you don't hear the picket fence effect while driving down the road at 70 mph either. If you recall, if you were in a weak signal area, moving even a couple of feet would cause you call to fade out, and gave birth to the infamous "can you hear me now?", this was due to reflected signals (multipath) arriving at the antenna slightly out of phase with each other and canceling, CDMA technology uses a "COMB" filter and actually uses all the signal paths arriving at the antenna strong enough to provide useable data packets. If you cell phone works inside an elevator then that building more than likely has an indoor repeater. We used to install them only if there were enough clients, but now it is common place since the equipment has become less expensive to do so. Also with the advent of roaming agreements you chances of having more than one provider in a city environment is very high. I have not kept up with E911 legislation but that may have influenced the in-building coverage as well.
Oh yea, I forgot, They will normally put an antenna on each floor of a building. We put several antennas inside of large manufacturing facilities.
