Well cant handle the fact that someone can steale a phone, but user may have a code on phone, dont see that as a problem...
By using BT , user also need to have BT enabled, then again BT only works on some meters,,
Dataconnection is the best for using here,
SMS is not ok,, can't lock the numbers out,
Call in , could be an answer, but again google prevent me from sending direct calls,
As it are today, the users just gets a code , and now many have the codes,
I know i cant prevent people from "joining" access together with Granted people, but most people i can lock out.