Need to find or build a phone mic amplifier for a person with odd handicap

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.
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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.

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Solution criteria:

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.
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A pre-existing product would be fine if inexpensive. (Say under $30. Not a hard limit though.)
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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.)
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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.)
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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.
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It should not require two hands to talk using it.

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Any ideas?

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