It's not unusual to install equipment above a dropped ceiling. Alarm panels and access control panels come to mind. But the thing is that with them they are powered remotely by a low voltage transformer plugged into a receptacle that is not within the ceiling space. With a phone system you are going to have a problem. As far as I know the only thing the NEC says is if the system is installed there you can't plug it in. You can't connect it to a receptacle installed above a dropped ceiling nor can you run the line cord through a hole in the tile to a receptacle below.

That said, the building code may have something else to say especially if the space above the ceiling is used for environmental air.

-Hal