Through some research and experimentation, I've found a possible partial solution:
After Apple replaced my iPhone 5 in November (I'm in the US, and my carrier is Verizon), the incoming audio for phone calls was noticeably better but still plagued by drop-outs, robotic-sounding noise, muffled effects, etc. as others on this thread have described. Last week, I found that changing this setting helped me:
Settings > Music > EQ > change setting (from "Off") to "Spoken Word".
I know this setting will change the tone for music/etc. but I need my iPhone much more as a phone than as a music player. For phone call incoming audio, the resulting change in sound/tone is subtle (and not objectionable). More significantly, I've found this change improves intelligibility when listening to the other party on a phone call, and it's now much less fatiguing for me to understand what the other person is saying. Also, other parties now say I sound a bit better to them; that is, the audio outgoing from the phone is a little clearer, too. While I use a wired headset most of the time, I've found these improvements apply even without a wired headset (although I haven't tried Bluetooth).
For sake of completeness, here are a few other thoughts I've found _may_ be valuable to consider:
1. During a phone call, be sure all 3 microphones are not obscured (there's one microphone on the bottom edge, one on the front by the speaker where you'd put your ear, and one on the back between the camera and the flash). So don't lay the phone flat on a desk, but instead prop it up to ensure the mic on the back is exposed to air. Also be sure the mic on the bottom edge isn't obscured; periodically taking off an external case to remove any accumulated lint would seem prudent.
2. If you're going to move the phone away from your head, be sure to use the speakerphone, Bluetooth, or a wired headset. If you skip all three of those options, be sure to orient the phone with the bottom
edge toward you. If you orient the phone with the display facing you squarely, so that the top and bottom microphones can both pick up your voice roughly equally, then the noise cancellation algorithm is likely to suppress some of your voice when you speak to the other party.
4. The noise cancellation algorithm can take several seconds (even 10 or 15 seconds according to some reports) to adjust to a new noise environment. When working from my home office, which is admittedly rather quiet, and having changed the EQ setting as describe above, I've found the algorithm does fine after just a few seconds.
5. Replacing the SIM makes to improve the audio makes no sense to me, and most reports I've seen (particularly on this thread) suggest it's a waste of time.
"Your mileage may vary" but I hope this change to the EQ setting may help others.
