Their point is that you can always give your number to anyone and they can directly use it to contact you. It's never unclear what number to give someone so they can reach you. Yeah calls have to be routed, but routes are implicit, not something you have to worry about as src or dest.
Say you're at home, you have a landline, and you don't know what your own phone number is. How do you get it? You call someone with caller ID, and ask them to tell you what number you called from.
Of course, I haven't had a land line in years and my cell phones tell me their numbers in the settings.
So like I said, you call someone to get your number. Like how you google what your ip address is.