phone up from a 'private' line like a big buisnesses do. ones that withold their number . get a mate to call so they won't know it's you. tell them they have got a free headset, mobile charger, or other promotional give away. check what type / model of phone they have to make it sound authentic. then ask for their name and first line of address and postcode, for security reasons of course. ( people are so thick they give this out all the time to anybody. ) and then you'll know who they are.
of course you mustn't do this for criminal reasons or to harrass some body.