can I port a number to a family member's cell-enabled watch paired using family setup to my iphone?

I'm considering getting a cell enabled Apple Watch for my dad, which would be linked with Family Setup to my iPhone (I also have a non-cell enabled series 4 Apple Watch). Wondering if it would work for a watch, say a series 4 or newer, to be his primary call device (vs. an actual phone, which he runs into trouble setting up, managing, adjusting ring volume, etc) since that's basically all he wants from a phone anyway, possibly with the exception of some fitness tracking and an occasional map.

Wish it could be linked to his iPad, which he DOES use successfully, but until that's possible, considering the potential pitfalls with linking to my iPhone,. We live across the country from each other, so I won't be around for everyday troubleshooting.