Cell phone use in the immigration and customs hall is absolutely prohibited. If anyone is a regular international traveler they know that.

I see "no cell phone use" signs all the time when I come back to the U.S. Best thing is to turn it off after you exit the aircraft. If you forget to turn it off it rings, don't even think of answering it. I once saw a person chatting on the phone at LAS customs baggage claim a few years ago and the custom agent yelled at the top of his lungs, "Turn off the phone!". This is not uncommon.