I work at a TV station. Usually we use the phone in whatever live truck we are using to provide the reporters and photographers IFB, which is what they listen to in their ear to get cues and to listen to the news program in progress. It consists of the director cues and the show program, but not their own voice because the delay in hearing yourself is really disorienting. That IFB audio is sent out to a special receive box they plug their ear piece into. It can get nice and loud to allow them to hear in all kinds of situations.
They have started to use from time to time their iPhones as IFB receivers (and dialers) when using those "backpack" style video transmitters, when no truck is available or it's impossible to use one. However, the iPhone audio through their ear piece doesn't really get loud enough. I have built a couple of cables to take the iPhone audio and drive a secondary amplifier box and then their ear piece plugs into that. I found some 1/8" to 1/8" mono cords about two feet long. I cut a couple of them in half, put an XLR connector on one end to plug into the amplifier box and put the 1/8" mono plug into the iPhone. It works, of course and gets nice and loud.
My question, is it harmful in any way to the iPhone to plug in a mono plug for a long time? I know that one side of the amplifier is shorted out with the mono plug, so is this doing damage to the internal amplifier, or is it smart enough to recover? They don't need both "sides" of the audio as this is just being used to listen to a phone call. Using the molded 1/8" plugs provides some durability that i can't match if i build a cable with a stereo plug, so I'd like to use these plugs if possible. Nobody is harder on equipment than TV reporters and photographers!