numbr provides free, anonymous, automatically expiring phone numbers that forward incoming calls to any other phone number. The service is useful for classified ads or other postings where you don’t want to post your permanent phone number, but would like to provide a way for people to contact you via phone.

“The desire to communicate can not be crippled by concerns about privacy. numbr unleashes the true potential of a global community by making it a safer place. Anything we build must be simple enough to be used easily” says the numbr factsheet.

When a user is given a phone number, that number is unique and never used again once it expires (unless you decide to reactivate it). This is a nice feature because it avoids the problem of people calling looking for the former owner (which is a problem on every permanent phone number I have or have ever had). numbr also offers built-in call screening as well as a built-in telemarketer filter.