Since you speak about the handset, be advised that one used to put a handset into a pair of muffs that was a part of a 110 baud or 300 baud modem - it was a way of getting around the direct connection restrictions that Ma Bell had at the time (1980s). There was a speaker (or similar) on one end used to generate sound, and a microphone on the other end to pickup sound. So much time has elapsed since those daze, it would be a challenge to find one of those old modems. OTOH, many phones still have a transformer inside for "sidetone" coupling of the two signals. So one can use a coil pickup for listening to conversations; some experimentation is in order to find the most sensitive position for pickup. It might be possible to use the same coil for transmitting signals to the phone; i have not tried that. These are the two possible ways to allow what you have in mind. Just do not record conversations; there are some fairly stiff federal regulations.....