My phone has two SIMs on two different networks, one for work, one is mine - does this mean I am two people when the census-by-mobile counts people? If I've also got my LTE tablet with me am I three people?
I really don't think this is going to be accurate enough to use for a census. You'd need to follow it up with more accurate measures to correct for errors, so you might as well do it properly in the first place. The census is supposed to be a count of people, not a rough guess.