You mentioned "Microphone and codec performance" as well as device
details (vendor, model, OS, etc.) but I'm wonder if you'll try leveraging
multiple microphones (on handsets that have them) to help eliminate
echo?
That might not be good for the phone. The benefits achieved by having the microphone slightly closer to your face are more than outweighed by the damage of leveraging the microphone out of position. It's probably best to leave that decision to the end user rather than try to implement it in software.