I did not factor in cell towers or anything like that for good reasons. If you call a phone number, you don't need to know where that phone is, you just get connected to it, and thus to the person holding it. That is the symbolic link, right there. Sure you can change the phone, but the number on the sim card stays the same, provided of course, you use the same sim card in the new phone. Changing numbers would be like cutting of your hair, as others pointed out.
I thought about it like Harry calling for Toot. He calls out his name and there is only one being in existence, that will respond to this name. Much like a cellphone, you don't get randomly connected to any phone, you get connected to one specific phone, and one phone only. I admit, the whole technical process might be different, but from a users point of view a phone number and the phone are directly connected.
Hexxing the phone might be an issue, that's true, but if you have nothing else to work on, it might be worth it.
While I myself would allow this, I would not point the players to it, they would have to think of this themselves.
Email addresses on the other hand would not be okay. When I call a phone, I get connected to the person on the other side, an Email gets send to a server and can be accessed from pretty much everywhere.