Okay. - I'm trying to help a friend with an odd handicap.
This person can only speak at a whisper and people at the other end of a phone conversation cannot hear her. This condition comes and goes but can last several months.
Her hearing is fine.
Existing phones (I know of) w/volume controls turn up BOTH speaker and mic which is a big problem.
Turning it up loud enough is way too loud.
Hurts her ears and causes feedback loop problems speaker to mic.
Ideally it works with a cell phone.
Suggestions for a standard phone are better than nothing but situationally that only helps her maybe 10% of the time.
A pre-existing product would be fine if inexpensive. (Say under $30. Not a hard limit though.)
Building it from scratch is an option.
I have an electronics background (way back there) but I wasn't a designer or engineer.
I can build something if given a schematic or repair/modify existing circuits.
(I DON'T want to modify a cell phone so a suggestion for a cell would have to be some lay-over the mic gadget.)
Should be easily portable and run on batteries if for a cell.
(Battery pack and wire or a circuit box and wire is okay if small enough I can adapt to a belt clip.)
There needs to be a simple way to disable (turn off, remove) the amp for those times when she isn't having problems or when someone else needs the phone.
It should not require two hands to talk using it.