I have the same issue with my iPhone 7 plus Matte Black 128 GB. I'm at my 2nd replacement phone for the same call sound issue when calling from the phone, not any other app using the mic is making this issue.
I tried something though and it seems to work for me, maybe it's going to work for some other people too. While the noise cancellation is activate (or not, it doesn't matter because for some reason the back mic seems to be always working...), simply put your finger on the back microphone between the camera and the flash and talk. It should be okay.
The back mic seems to receive the voice whether the noise cancellation is on or off and that must be why our voice is getting cut while talking on the phone but not anywhere else. I guess it must be some kind of calibrating issue (probably within the Phone app) itself, considering it's not making it with other app.
I'm running the latest Beta software and the issue is still there, I sincerely hope Apple is aware of this issue (and it doesn't seem to be physical considering no other app is having this issue.
Tell me if it works for you too, I'm curious!
We'll at least have some sort of workaround until it's permanently fix!