Check out GrandCentral.
Recently acquired by Google, Grand Central provides a phone number that rings all your phones, digital voicemail, and a number of other features - free.
GrandCentral allows you to:
- Screen calls
- Record calls
- Block calls
- Receive voicemail notifications by email
- Accept calls from the Internet without publishing your number
- Switch phones during a call
- Click to call numbers in your address book from a phone with your Grand Central caller ID
It's currently in Beta test mode - which means that Google may decide to charge for some or all of these features in the future. In the mean time, it may be worth checking out.