There is an iTunes app called Ring Shuffle–iTunes page and web page–that pretty much allows you to create a fake phone number to use on an ad, give it to someone, etc masking the main number.
Their website even promotes for customers to use it when contacting a car dealer (thanks) but this may also be useful for a dealer to mask their main phone number or cell phone number so it can be used in websites like Craigslist, promotional items, etc.
