iPhone keeps ringing even though I answer it

Greetings,

I own an iPhone 7 Plus 256GB.

Last week i experienced a very weird problem that when the phone rings I answer it, the screen turns to normal talking screen with all the icons of numpad, speaker etc but the phone keeps ringing in my hand and does not actually pick up. The issue gets solved after I turn the phone off and on. But after a while it happens again. And since then this is happening all the time.

Also when I want to record a video the camera simply do not pass to video mode. It gets stuck the screen goes black and I need to close the application. Same happens when I try to record a video in whatsapp application. Once again the issue gets solved with off and on. But happens again after a while.

So I thought well I'll simply keep the old corded headset connected and will answer calls through it. Well guess what. Nothing happens when I connect it. There is no problem with the port since the phone charges without any problem. But when I connect the headset simply it does not recognize it. I know the headset is fine. I checked it on another phone. And when I say does not recognize it is not only the microphone. It acts as if it is not connected. No music no nothing.

All the apps are working fine. I do not have any games whatsoever installed. In total I am not even using half of the 256gb capacity.

I always close all apps fully by double clicking the home button as a habit. There are NEVER any apps running in background. Not that I know of.

The phone is up to date. Currently iOS 15.3 is installed.

The battery has recently been replaced. It is at 100% health and phone is at full performance according to battery information in settings.

(Though I should say here that the battery has not been changed by official Apple store since they have rejected doing this. I purchased this phone from Japan and the Apple store in Turkey simply does not give any sort of service to my phone even if you want to pay for it. The official resellers also follow the same path so I had to make this replacement in a small unofficial shop since my battery capacity dropped below 70% and battery info was saying that the battery was not sufficient for full performance anymore and it was not holding charge for even a couple of hours. It has been 2 months since this replacement.)

Currently my iPhone is unusable as a phone. I have to turn the phone off and on to simply be able to make a call. Apple store will not help me or give me service because I purchased the phone from another country(which is completely legal by the way). So community, please help me.

Cheers,

Cagri

[Re-Titled by Moderator]