Elgoog AFAIK, the builtin phone and messaging apps don't do much. Since there's no SIM, there's really nothing you can do with either (the built-in messaging app is SMS-only, which is unusable without a SIM). I don't think I've ever even launched them.
Now, on an iPad, you can piggyback on a linked iPhone to make calls and use SMS. I'm actually not sure if this is possible on a Pixel Tablet/Pixel Phone combination, but I've never heard of it. Personally, I only use this tablet for light web browsing and ebooks, so I don't even have Play Services installed (which would almost certainly be a basic requirement for something like this, as you'd need to be logged into some common account between the phone and tablet to share the SIM).