For instance, the OP, the Grandma doesn't want to know the kid's number. She wants to know the best or most convenient way (from her perspective) to have a voice call with the kid

I don't think that's right. Grandma knows how to find his contact to text him, she should be capable of finding it to call him.

I think she actually wants his phone number for something. Maybe she wants to give it to the cute girl in her church for him, maybe she wants to sign him up to a spam call list. Who knows, but I find it the more likely option.