Back in the day, if you knew the secret menu, you could change the default vocoder to use on the network. The cheap cell company I used defaulted to half-rate. I would set my phone to the highest bitrate, with a huge improvement in quality, but at the expense of the towers rejecting my calls around 25% of the time. When I would call landline phones, people would mention how good it sounded.

For a short while Verizon enabled this on our super grandfathered plan. Well not exactly this, it was some more modern codec dubbed HD and it sounded so good it was freaky and unnerving