How to mute the telephonic communication sound?

I know how to mute the phone, including the media sound,

but I don't know how to mute the telephonic communication sound.

If I press the volume down button again and again while I am calling,

the magnitude of the communication sound goes to the minimum, which is not zero.

I want to mute the phone call conversation sound completely.

Is there any way to do that? I don't care whether the method requires rooting or not.