I’ll have a go. Your questions are kinda loaded.
Here’s how I understand it.
If you’re conducting a poll then for me:
Phone rings. I can hear the ring. Of course its volume can be adjusted. I use the phone regularly (speaker to ear, mic near mouth). I am fortunate to be still able to hear the phone naturally. Of course the volume can be adjusted. Probably if I handed the phone to an able-hearing person it would blast them out but whatever.
As for options:
One HA can be streamed to by any modern bluetooth phone. In mono. I think it plays the phone to the other side as well. The phone can be answered by the HA or you can answer the phone. The HA can be controlled by the phone as well.
Many HA’s can be streamed to by an iPhone using an Apple specific variation of bluetooth. Except for the HA mentioned above you would have to hold the phone mic near your mouth. You would have to answer the phone on the phone. It can ring in your HA’s. The HA can be controlled by the iPhone.
Most HA’s offer a necklace (neckloop) device that can control the HA’s and be streamed to again via bluetooth from any modern phone. Or cable. I think it goes to the HA’s via FM. Not sure of that detail.
The “coil” you mention is a whole 'nother animal.
Many HA’s have this thing called a telecoil or t-coil or as you say just coil. It uses an electromagnetic term called induction. You activate it on your HA’s and then walk inside what is called a hearing loop and if something is being amplified across the hearing loop then you will hear it in your HA’s. The “sound” is not audible without a telecoil.
You can also use a personal hearing loop also called a neckloop. Same idea. You output from some kind of amplifier to the neckloop that then your HA’s can “hear”. The personal neckloop may be connected to via bluetooth or cable. It can also operate your HA’s and phone.
All hearing loops are inherently mono. You can hear it in both HA’s but the signal is mono. After market neckloops do NOT use FM. They use induction.
I’ll let someone else fill in the details about the magnet for use with a phone as that is not something I’ve gone looking for. But as I currently understand it…yes. If your HA’s have a telecoil and if they’re programmed to respond automatically to a magnet in their immediate vicinity (ie. a phone with a magnet near its speaker and held near the HA not your canal) then the telecoil turns on. What happens then I’m not sure. Does it play the sound to both HA’s? I guess so but I’m not sure.
Wanna just control the HA’s from a phone? Many HA’s offer programs for the two main mobile OS’s that can control the HA’s. ie volume, programs etc. The one that I know of does it by a high pitched sound signal.
Even more simply, many HA’s offer a remote “clicker” to do the same thing.
That’s how I understand things currently.