Screen does NOT turn black during calls.

Had the same issue on my iPhone 7 but that was because I set "Auto-Lock" to "Never". Check Settings, Display & Brightness, Auto-Lock. If set to "30 Seconds", guess what, the screen dims after 30 seconds 🙂

Do you have a case on it? If so, then remove it and see if it helps.

If not, then try a reset.

If that fails, then try a restore.

If that fails, then make an appointment at the genius bar or call Apple support. If it is defective, then they will replace.

I am having the same issue. I have the problem without the case on it. I still have the problem after both resetting it and restoring it. Also, my phone fails to connect via bluetooth to my car consistently. These are annoying problems, as I've accidentally face called/texted several people during other phone calls. Super embarrassing!

I had the same problem until 5 minutes ago. I fixed it by going into settings> general> reset> reset all settings. Click yes..... Then phone reboots. When going through the set up don't worry all you data is still there. I clicked restore as new iPhone. Rather than downloading old back up. Just going to do a fresh back up now. Anyway hope it works for you!

If you have a protective covering (like an anti glare screen) on the phone that has been on for a while, it may need to be removed. Mine was getting dirty around the sensor and wasn't recognizing that I was on the phone. Once removed it worked perfectly.