What is wrong with the sound on my iPhone 7?

Okay so this is a confusing one so I would appreciate any help at all.

Over a month ago, I received a phone call but when I answered I simply couldn't hear the person on the other side. It happened again when I tried to ring back, so I turned my phone off and on to see if it would fix, but it took around 5 minutes for my phone to turn back on, it simply only had the Apple logo. When it turned back on, I realised all sound on my phone was broken - no ringer, no Spotify/Yourube, I couldn't make or receive calls.

Upon talking with Apple, they didn't know the problem so they said I would simply have to send it for repairs, but due to the price I decided to hold off. For a month I had to go without sound.

Fast forward to just over a week ago, I turned my phone off and on because I hadn't for a month, it turned on immediately and the sound was fixed.

5 days ago, the same problem occurred with the phone call. I turned the phone off, the sound went off. I turned it off and on twice and then the problem was fixed.

This morning, the problem occurred again, but after turning it off and on multiple times, waiting 5 minutes each time for the phone to reboot, the problem is still here. No sounds, no calls. Nothing.

Does anyone know what the problem is or how I can fix this?

Isejj