During calls, the phone turns off

Hi Marwan,

It sounds an awful lot like the proximity sensor in your phone has died. It's primary function is to tell when you're holding the phone up to your face during a phone call in order to blank the screen when you're obviously not going to be looking at it. Depending on how it fails, it may be telling your phone that you're holding it up to your ear even when you're not, thus the blank screen during a call.

Replacing the proximity sensor would be the first thing I'd try in attempting to fix the problem. You can pick them up on AliExpress for about $5 USD with shipping; they're available on most of the major retailers like eBay and Amazon as well - pick one that works for your location. Here's an example.

For Xiaomi Redmi Note 9 Pro 9s 8 Proximity Ambient Light Sensor Flex Cable Replacement parts - AliExpress

Of course, it would be nice to have a guide to show you how to replace it, and I'm sorry to tell you that iFixit doesn't have one for you, but there's a French site called SOSav that does.

Proximity sensor Xiaomi Redmi note 9 Pro repair - Free guide - SOSav

You generally test a proximity sensor by placing your finger over the part of the screen where it's located while you're on a phone call; when it's working the screen will turn black and when you remove your finger it should turn back on. On your phone it's just to the right of the front camera. Give that a try, but I suspect we already know what you'll find; that it doesn't do anything since the screen goes black right away. You could try the opposite approach and see if you can get it to turn back on by shining a flashlight at it when the screen is black. If it's got any function at all, that might turn it back on, but still means it needs to be replaced.

That's what I'd try if it was my phone. The part is inexpensive and it's not too bad of a repair, although it is somewhat involved since it's under the motherboard. At least you don't have to take the screen off! Oh, and don't forget to buy some precut rear cover adhesive so you can resecure the back of the phone when you're done.

Good luck with your repair; let us know how it goes!