How does a phone call by a hearing impaired person work?

Does the person on the receiving end of a phone call by a hearing impaired person talk louder, turn on a loudspeaker,or what?

it depends.

phones need to be hearing aid compatable, that is, the coil in the hearing aid is activated by the coil in the phone receiver.

If the person is totally deaf, then the phone is TTY or Teletype.

A TTY operator intercepts a voice call and types to the deaf person, the deaf person types back and the operator voice to the caller.