Get caller ID so you can pinpoint exactly who it is, Its up to you then what you do 1 ring the number confront them, 2 check the number in the phone book and get an address then go round and have it out with them 3 inform the police of your findings and they can investigate it with the phone company and check call records, If this is a prank from a so called Friend i would confront them and if it did not stop then go to the police.