For those of you unfamiliar with the concept of a true name, basically, if you knew the true name of a spirit you could use it to invoke said spirit and have power over said spirit. An example you may recognize is that of Beetlejuice, saying the name "Beetlejuice" thrice invoked him or dismissed him and so on and so forth.
It occured to me recently that a cellphone number is very much conceptually akin to the mystical concept of a true name. If you have someone's cellphone number you can call them anywhere and theoretically invoke them as well. You are granted power over an individual by being able to contact them at any time. Also, just as with summoned demons, if you invoke people for stupid reasons they will take it out on you.