I have a theory. Clementine's parents were in Savannah when the outbreak happened.. Oberson (the leader of Crawford) quarantined Crawford from the rest of the city.. he recruited people that he felt were strong enough to survive the outbreak.. Clem's parents were amongst the people that Oberson recruited.. when the walkers started to take over Crawford Oberson just disappeared out of nowhere. I think Oberson is the one that's been talking to Clementine on the walkie talkie. Why else would somebody have it? What are your thoughts?