Sometimes the phone turns off when the call is connected

If it doesn't then a downloaded¹ app might be the cause of the problem. The trick is to find out which one. You will need to uninstall each downloaded app, one by one and test the phone in normal mode between each uninstall, to see if it works OK or not. If it does the last downloaded app that was uninstalled was the culprit.

¹ A downloaded app, is one that you downloaded and installed and didn't come preinstalled on the phone.

It might take a bit of time doing the above as the problem is intermittent.

If it still occurs in safe mode, if possible try using the phone with the charger connected (and turned on) to see if it happens.

If it works OK with the charger connected then it might be a battery problem.

If it still occurs with the charger connected, try a factory reset and check.

Be aware that a factory reset will erase all your data from the phone. It will restore the phone to its factory default condition.

Important: If you decide to do this, backup the phone to a computer or the cloud before performing the factory reset. If the phone has a Google account associated with it, you will need to know the user name and password for the account. This is so that you can access the phone to set it up again, after the factory reset has been done, due to the FRP (factory reset protection) Lock that has been enabled by the reset.

Once you have a backup, factory reset the phone and then check if the problem still occurs or not.

If it works OK after the reset, then restore the phone using the backup you made before and then check if it still works OK. Allow some time to make sure it is working OK. If it does then the reset has resolved the problem.

If the problem still occurs after the reset then there's a hardware problem and the phone will need to be opened and tested further to find out what's wrong.