alt Hacker Newsthat hack I've wanted to do is to set up myself as a phone company and then be able to give out different phone numbers to different people so that if only one person has that phone number than I know where that phone number got leaked from. if it is being handled digitally and everything gets forwards, calls to 0001 are from my family, 0002 is friends 0003 is work, 0004 and up are given to each business that wants to contact me. If 0042 gets called by a company that isn't the one I signed up 0042 with, then I know their database leaked.
There aren't enough phone numbers to make this work. There are only ~5.7B phone numbers in the NANP that can be assigned to individuals [1] and these countries have >400M people, so it's just ~14 phone numbers per person. Which works if we each have a couple, but not if we start handing out many per person.
[1] 10B ten-digit numbers, less prohibited ones like area codes starting with 0 or 1, the 555 exchange, etc.