For a couple of years now I've been using [my area code] - 555 - [a unique 4-digit pin] for dealing with otherwise "walk-in" business that look at you like you have three heads if you decline to say a phone number out loud. In the US at least, the 555 block is defacto "fictional" and typically isn't assigned out, so it lets me create a valid looking number that won't accidentally ring some random real person if they try to call it.
Minor quibble, 555 isn't fictional, it's typically reserved for teleco internal use, or at least that was the case 25 years ago, who knows what the deal is now. Used to be you could wardial 555-xxxx and end up with all kinds of weird AT&T field installations, back office numbers, switch remote command and control modem numbers, etc.
