Yes, I am also having the same problem. I got the iPhone7 Plus about a year ago and almost immediately started getting complaints from people I called (or that called me) that they couldn't hear me very well. Comments ranged from "you sound very distant, or "are you in a tunnel", to "are you holding the phone away!" I have had the phone replaced already but I still have exactly the same problem.
I tried turning of Noise Cancellation, but it made no difference.
Like other people on this discussion, I use an Otterbox Strada case (you want the best protection for a $900 phone, right?). When I take the phone out of the case, it does seem to improve things, but not 100%. So there must be a connection between this Otterbox case and others like it. Does it block one or more holes that are necessary to the good working of the microphone? Who knows, but neither Apple nor Otterbox seem to be very forthcoming with any advice, which is very disappointing. I have a case open (no pun intended) with Otterbox to see if they have anything useful to add, which I will post here as an update.
Other than that, I feel like I'll have to go for another replacement, which means I'll have to go back on my iPhone6 for a while. But that's not all bad... at least people will be able to hear me!