Its not free if you value your privacy, Google collects and stores all your information.

If you're on the internet "They" have all your info anyway.
In this day and age, if they want you, they will find you.
Google may collect information, but if an agency wanted to know what you're up to, they can monitor Any of your communications.

I've been using something similar on my iPhone for quite a while.. I give out my google voice number to people I don't know that well rather than my actual cell since it's super easy to block people since my phone doesn't natively have that capability ( I do with a jailbreak app though ) ... the best thing for me about google voice is the voicemail features.. they transcribe voice messages into text ( not always great at it ) .. but you can also have multiple greetings based on caller id .. if someone has a blocked number for instance I have a greeting that tells them I don't accept calls from blocked numbers and then it hangs up.. for people who annoy me I have the onision "I'm a banana" song play clear through.. etc

=)