Here is what I figured out. You cant hold the phone between your shoulder and ear as so many commonly do when doing something while on the phone. You cant hold the phone tight against your hand at the top of the phone near the camera. You cant use a portfolio case of any kind with the cover opened back. Disabling noise cancelling will not correct the problem. This is 100% an issue with the rear mic by the camera.
How to use your phone until Apple corrects this issue.
1: Hold the phone with the tips of your fingers towards the bottom of the phone creating an arch in your palm to allow plenty of room between your phone and your palm.
2: Do not hold the phone with your shoulder while on a call.
3: If using a portfolio style case. Open it to answer the call or dial, then close the case and make the call with the case closed making sure you hold your phone as listed in 1:
4: Put it on speaker phone
5:Bluetooth device.
This is the only way to use your phone with decent audio quality until Apple corrects this problem via software, firmware, hardware update or combination of all three. Exchanging sim cards, phones and other fixes will not correct this issue.
If implementing 1 through 5 does not correct your problem then you may need a phone replacement/repair, but I would be willing to bet that almost all phones will work properly if following those techniques. At least using these techniques you will be able to make an important business call without missed communications and sounding completely unprofessional.
Brad Mitchell
software/hardware/networking for over 20 years