I am not an expert on this, but let me share my ignorance. Acoustic Phone uses the mics of ONE of the HAs (assuming you have a pair) and sets up the settings for hearing the receiver of the phone up close and feeding it in your HA.
Duophone takes phone audio and feeds it from one ear to the other. If you have telecoil, I believe that could use either ear to grab the audio via electromagnetic signal emanating from your handset, and gives that to you in both ears. Or your Acoustic phone will be piped to the other ear.
Acoustic phone is a manual setting. I have to either go through the long presses on the HAs or use the app on my iphone to change to that program and then change back. If your HAs have telecoil, that needs to also be programmed for use by your HCP. Activation for a call isn’t anything I know about at this point.
My acoustic phone setting works OK. I have it for when I’m handed someone’s phone and told, “Here, talk to xxxx!” I have on a couple occasions given up and said, “I just can’t hear you on this phone, can you call me on mine?” as my iphone streams SO much better into my ears than the acoustic works. But it (acoustic phone program) is better than a sharp stick in the eye.
WH