I suspect the companies have a pretty good idea of how well their networks actually perform in service.
However, according to the cell phone articles in I&T magazine (previously posted), a part of the problem is the ever higher consumer demand for cell phone service and the finite space of radio frequencies. The more 'tricks' they do to compress signals and squeeze in more conversations the more risk there is of trouble. With the coded digital transmission on a common frequency, if there happens to be no available space, the call becomes hung. (I&T said calls are interspersed between silent periods of conversations.)
I don't think they anticpated the massive demand for cell phone service. It was one thing for business people to get and individuals to get it "for emergencies". But now families give a phone to all their kids, even young ones, and the texting and voice consume channels. (Apparently the data (all non-voice) transmissions can be sent on a delayed basis which makes them easier to accomodate.)