I like to give obviously fake numbers. Like 12345 for a zip code or 212-555-1234 for a phone number. Most people don't care enough to have a reaction, now and then you get a laugh, and rarely you'll get someone who calls it out as bogus. My standard retort is somewhere between "Are you saying I don't know my own phone number?" and "Are you calling a liar?!" depending on how surly the response.