If you want to sell something on Craiglist, eBay or Amazon your possible customers might want to speak with you. If you mention your phone number on the advert, it might stay posted there for ever and a quick Internet search for your personal phone number will find it easily together with the text which carries personal information about what you do/sell/think.
Telemarketers could also gather your phone number and annoy you with nuisance calls attempting to sell you garbage, the reasons why your phone number should never be made public are many.
Google Voice can also be used to hide your phone number using a Google number to receive calls but it can not be used as a disposable weblink like Babble.ly does and Google Voice does not allow for a link to be directly posted to a website, in order to connect the calls Google Voice uses a hard to embed flash based widget.
If you want to use extra features like custom caller greetings or number blocking, there is another similar paid for service named LetsCall.me.