Home phone telephone Easter egg: calling your own phone line?

[rowrrbazzle]

An old thread on the ringback number.

Phoning Yourself

Once, many moons ago, I read somewhere that there is a certain number you can dial on your home phone that will cause your phone to ring if you hang up immediately after dialing it. I believe it can be used to test the ringer on the phone. I tried it, and it does work, but I can’t for the life of me remember what it is. Anyone know?