I too discovered that I was suffering problems with outgoing call quality on my new iPhone 7. I don't use my phone for calls that much, so it took about a week after purchasing to discover that my voice was sounding all muffled, like the phone was wrapped in something. I did a couple of tests phoning my own office voicemail and sure enough it was fine when I switched to speakerphone, but sounded awful when I held the phone to my ear. I did the usual thing of checking the forums to discover I was not on my own and was ready to make a Genius bar booking.
However, I decided to do a last-minute check and entirely removed my phone case. I discovered that upon testing, call quality was much better. Equivalent to using speakerphone. I'm not saying it's great compared to other phones, but certainly not muffled any more, and probably not that much different to my iPhone 6.
After a bit more testing I discovered that it was actually the phone case's flip cover that was blocking the rear microphone (between rear camera and flash) when folded back on itself (as you do with this type of case).
I had almost exactly the same phone case with flip cover on my iPhone 6 and never had this problem, so there is obviously a change in the way that the microphones work for phone calls (but not Skype et al) on iPhone 7. This was only ever a temporary case, so now I've purchased one (bumper style) without the flip cover and have a screen protector instead.
I'm not saying this is the solution for everyone, just that it's worth checking that you aren't somehow blocking or covering the rear microphone.
Regards,
Kris.
