Same problem here, been busy with it for weeks now. iPhone 7 black 256 GB on Vodafone network (NL)
I experienced from the beginning microphone cutting out during calls and/or at times people could hardly hear me like I was under water or standing in a different room. With a little movement away from the mouth and speaker still to my ear, the microphone randomly cuts out and/or drops. My clients have told me quite some times that they could not hear me temporarily or for longer or very badly. Very troublesome and annoying actually. I am using roughly every new iPhone model since the 3G model. Never had this problem with any of them.
1) 1st I waited for updates as Apple announced shortly after some software improvements/updates for iPhone 7 / 7 Plus (no stress yet, it's Apple! they will for sure fix it 😉)
2) After all updates and resets and new install efforts & resets (really absolutely anyting that is recommended for problem solving) -> mic cutting out randomly and/or dropping during calls remained
3) Called Vodafone service and they answered me that at times during the call they had difficulty hearing me ... exactly!
4) Vodafone picked it up under warranty and checked it, returned without improvements -> mic cutting out randomly and/or dropping during calls remained
5) Vodafone store renewed SIM card -> mic cutting out randomly and/or dropping during calls remained
6) Called Apple support and they answered me as well that at times during the call they had difficulty hearing me ... again exactly the problem! They advised to make an appointment with the Genius Bar.
7) The Genius Bar of Apple Store in Amsterdam renewed the iPhone 7 -> but unfortunately with the new phone still the mic is cutting out randomly and/or dropping during calls
8) Again at Apple Store in Amsterdam last weekend I was advised to do a DFU installation -> mic cutting out randomly and/or dropping during calls remained ... AND I had to re-install a lot as I was advised not to do a back-up from my iCloud backup. In hindsight I am not sure if that could be the problem at all, but when all other fails ...
I still have no solution. And still an iPhone 7 that is doing a bad job at phone calls. Burned a lot of time and energy on this matter. I have again booked an Apple Support call at 16.00h today.
Loosing a little bit my trust in Apple. Though wish not. I am actually a big and longtime happy Apple user.