Us older guys who spent some time on tractors and other loud stuff generally have some hearing loss, and cell phones are especially tough for me. I've found that if you tell your caller to put it on speaker, and you do the same, it will be clearer. You may have to tell them to speak softer, or further away from the phone, so it does not overmodulate.
I have a dumb phone, too- don't need all the foolishness that comes with a smart phone. I just use it to talk to people.