When someone call may Phone gets off

This usually indicates a problem with the proximity sensor. That's the sensor at the top of the phone that detects when the phone is up against your face during a phone call, at which point it disables the touchscreen and turns off the display. A defective sensor will cause those effects whenever you get a phone call, exactly as you're describing, and since the touchscreen is disabled, you can't do anything until the call ends.

First thing to do is make sure nothing is blocking the sensor; if you have a case on the phone, try taking it off, and if you have a screen protector you might want to try removing that as well. If those don't help, you may have to replace the sensor.

Luckily, on your phone that sensor is on a separate flex cable that can be replaced by itself. Sorry to say, iFixit doesn't have a guide for that repair, but the Spanish site Nadie Me Llama Gallina has the exact thing you need. As you might guess, the guide is written in Spanish, but the site does have an English language option. Of course, if you use it with a browser like Chrome that does language translation, then it doesn't matter what your native language is.

Manuales / Vivo X21 / Front sensors - Nadie Me Llama Gallina