prank called

there's a telephone in my apt but I don't know the phone number for it. I've only used it once when trying to report my lack of hot water to the apt manager.
today I received a prank call on that phone. the people on the other end were young and giggling while trying to ask in English if I wanted to be their friend. I asked (in Chinese) where they work and the said they were "sales staff" -- which I assume means telemarketers.