Well, an on-call doctor could have a beeper, it goes off, they hit the locker room and call who they need to call. Same deal with on-call responders. If you're a businessman with such pressing matters that take precedent over your workout, take care of it, then go to the gym......the only other justifable reasons is if you're wife is expecting a baby or somebody close to you is sick (even then you probably wouldn't be at the gym, and anyway the conversation is a few seconds and you're gone). Point being, legitimate uses for a cell at the gym are NEVER an annoyance or problem, it is the other 99.999% of the times people use them.
Most conversations are people wasting space calling their friends to make plans for a night of drinking, bragging about the previous night's drinking, guys fighting with their girlfriend, guys placing bets on games, or guys who aren't all that important trying to play the role of "president and CEO" to impress any girl within earshot, and just generally giving me the urge to shove the phone up their ass sideways. There is absolutely no reason a grown man or woman shouldn't have the attention span to concentrate on their workout and put the phone away for an hour or two. The only thing more annoying is cell phones in a movie theater.