Grandma writes down her grandkids numbers into her address book that she's used for many decades. It worked when she had a rotary phone, it worked when she had a cordless phone and it still works when she has an iPhone. If she ever decides to get some other brand of phone she will have no problem calling her gandkids because she has a system that works independently of whatever new technology comes around.

Meanwhile her grandchild will find it difficult to adapt to a phone from another company because they'll have to figure out how to export their contacts and import it into the phone, and then adapt to a different interface. They will likely just continue use phones from Apple no matter how much the charge for them, regardless of their reliability or quality. Too much hassle to change.

Now it's debatable which way of using the phone is better, but there are some real advantages to how grandma does it.

So just give her the damn number, so she can write it in her address book and she can dial in that number when she wants to call you. Her way of using a phone is not necessarily worse than your way of using it.