Back in the day, almost all cell phones had a miniature coax port on them. They were "hidden" in that you probably had to pry off some nondescript part or open the battery cover up or something.

This is how we tested mobile networks. It works great. The only problem is when someone forgets to make a tight connection or attach a signal attenuator. Since the first thing you always test is emergency calls, once in a while your call ends up on the real mobile network and the fire department shows up in your parking lot. This happened about once or twice a month where I worked. They knew what our business was so they were never mad at us, but we had to pay a fine each time.