AFAIK, there is no option on a Pixel (1, 2 or 3) to mute the sounds while on a call.
I have a Pixel XL (gen 1) and I had been searching for this answer for months. I then got "smart" and since I already had Tasker, I just set up a task to set the phone to mute/vibrate when "in call" then restore the sounds when the call ends.