We ran out of numbers and now have a few different prefixes, but the historic telephone prefix here in Woodland (formerly Yolo City) is 666. I was a little bummed that I couldn’t get one for the shop, as it makes you seem like more of a local fixture (speaking from a business point of view)
I always thought it was funny that the cross stitch floss called Christmas Red was color #666.
I read a Robert Heinlein SciFi book that conjectured it was 6^6^6 or six to the power of six to the power of six. Which is a HUGE number