apple watch with a different phone number

I know watches get a number so they can share a line with a pairing phone but in this case I want to pair my sons watch with his dad’s phone (he doesn’t have a apple watch) with his own different number (watch has cellular) so can has access to apps that are not available under family sharing. can i do that? i called apple support twice and seems like we don’t understand each other? one told me to call the app developer -{

thanks