Steam needs to figure out a better way to do this kind of stuff. My brother recently lost his prepaid phone so I just said screw it and got him onto my AT&T contact. So he got a new phone and a new number.
Well steam guard was tied to his old phone and if you change phone number the ONLY way steam support will reset your steam guard is by taking a picture of your photo ID, and the CD key for the FIRST game you bought on that steam account.
For him it happened to be the counterstrike 1 anthology from ~2005. Now in 2016, over a decade later, he's supposed to find the CD key and take a picture of it to prove he's the real owner of the account. With over $1,700 worth of games. And this is all because he switched phone.
He thankfully found the CD key in the closet, but if he didn't have that I'm not sure what he'd have done as valve support was adamant it MUST be the CD key, nothing else would suffice.