That was my thought, too. It is well known that the parents were listening to the messages (otherwise how would the mother have known that messages were being deleted?).
Still a bit of a fail for the police: a) why did they let the parents keep the phone (it was a means of gathering evidence/intelligence relevant to the missing girl); b) if they had reason for (a), why did they not think to protect the messages? - even in 2002, some members of the police force (sorry, "service") had dragged themselves sufficiently out of whatever slime it is that produces coppers to have mobiles of their own.