Sound quality worse on iphone to iphone calls than landline to iphone

When I received a call on my iPhone from another iPhone, the call was largely unintelligible, so I called the caller back from a landline phone and the call quality was much better. It's been 10 years since IEEE published my article explaining why cellular voice quality was poor. https://spectrum.ieee.org/why-mobile-voice-quality-still-stinksand-how-to-fix-it Is the main problem still the quality of the cellular network? Can Apple do anything to improve quality of calls between iPhones.