As of March 6th, 2012, we require all Zillow Rental Manager users to verify the phone number associated with their listings upon activation. Once that number is verified, you will not be asked to verify it again. If a new number is used, it will need to be verified as well.
We added this verification process to help reduce scam listings created on the site.
We do understand this is a hassle for our legitimate users, but hope everybody can also understand the need for additional measures against fraud.