A couple of my friends have recieved calls like this, one of them actually spoke to the guy, he was inviting him to join ISI, and provided he had a dual nationality i suppose it was legitimate. They asked him to come to a specific place at a specific time, which he of course refused, after which they then went on to ask about his car, registration plate, address ( he recently bought a new honda city) and his routine (gym wagaira ja rahy ho, sett chal rha hai sabb kuch etc etc)
as for the other friend, his brother or something left the country to study abroad while working for Nespak or some other government agency, without taking proper leave and clearance. He claims he had people following him on the bus frm lahore to isb, and other cases of his current number being suddenly blocked and being issued somewhere esle, like kissi ne ussi k naam pe ussi k number ki simm kahin aur niklwa li
Nothing to worry about, as long as you are clear and not involved in 2 number kaam as they say.