Had the same problem with my iPhone 7 for five days it was getting really annoying not hearing anyone calling me. I had a T-Mobile retailer look at it. He didn’t say much said it might be a hardware problem. I thought I’d give it a try to fix it myself. So, First I deleted all my saved Bluetooth. Then, I updated my phone to the newest one. (I believe my previous update is what caused the speaker to be grayed out while on the phone) With the update and restarting it, my phone is back to normal and working great!!! Can’t believe it worked!