first of all clear your mind as it's not mobilink's fraud.
i tell you the whole scenario. that person must have some contact in mobilink customer service centre and when he realized that you are not gonna sell your sim to him, he used an illegal way. he must have contacted a person who have rights to access your sim information and from there he cleared every thing and registered your number in the name of person who has your sim right now.
i know all this thing as one of my friend's cousin work in ufone customer service centre and he do all this nasty things.
so this is not any network company fraud but there employee is behind all this game.