I was going to say this. People don’t realize the degree to which this is true. The fidelity of old desktop phones was very high.
There’s also the fact that the receiver of an old desktop phone was positioned right under the speaker’s mouth. A cell phone receiver is not ideally placed. And you either have to put the whole phone against your cheek, which often causes accidental pressing of “buttons,” or hold it slightly away, which affects fidelity, and is also tiring. As a result, a lot of people use the “speaker” setting on cell phones, and this reduces fidelity even more.
If you use an old-fashioned desktop phone when you can-- at home or work-- you can improve what you hear even when the other person is using a cell phone. If a cell phone reduces fidelity by, say 20%, it’s additive, so when one party uses one, your fidelity is reduced by 20%. When both of you use one, it’s reduced by 40%.
If you don’t want to use a desk phone, and want to stick with a cell phone, you can use a headset, or get a speaker for it. Most cell phones are capable of producing better sound than their own built-in speakers can reproduce.
I know for certain that my hearing is very good, but I still have trouble understanding people when we are both using cell phones. Then I realized that when I used my phone in the car, where it used the car’s speakers, it was very clear. So I started hooking it up to an external speaker any time I knew a call would be more than a few sentences. Suddenly, the calls were perfectly clear.
So there are a few options: external speakers, headsets, desk phones.