Clint Eastwood's Character gained the upper hand on his adversaries in the movie The Good, The Bad and The Ugly, by telling them that a treasure was buried in the grave of Arch Stanton. This was a clever way to misdirect them away from where the treasure was actually buried, in the grave marked "Unknown" next to the grave of Arch Stanton. There is a modern application that can provide a similar benefit.
The real benefits to Google Voice in terms of privacy are the ability to carefully craft how you reveal your physical location to others. Especially if you require significant telephone contact with other individuals, there are many creative opportunities for cloaking your true location and residency with Google Voice.
You can choose to have a phone number in any US area code, no matter where your physical phone service is based. This gives the impression that you are either calling from that number or that someone is calling you in that area code when you receive calls or give out your phone number.
If you wish to convince someone that you are not a resident of New York but give them a New York area code, it takes a bit more convincing than if you gave them a phone number with an Albuquerque area code.
You can also forward calls easily to any phone in the US so if you normally reside and work in Phoenix, but have recently re-located to Indianapolis, none of your phone contacts need to be aware of the change.
Another benefit is the unlisted number. Because Google Voice is monetarily free for the most basic services, and because all you need is an anonymous gmail address to use Google Voice, there is no record of your actual identity. You can even have the calls forwarded to any phone, even a phone that is not registered in your name. These are just a few of the privacy benefits available through Google Voice.
As with any communication, your privacy is only as good as the ability of the other party, and any parties used to make that communication, to keep a secret. In 1569, it was common for political rivals in England to intercept messages through dishonest or scheming messengers, revealing the contents of sensitive communications.
The internet provider or phone company is the modern electronic equivalent of the messenger which, if not trustworthy, has access to your communications and might reveal them to your “political rivals.”
Like almost all phone companies, Google voice maintains records of calls made and to whom they were made. Because Google Voice offers so many different products, such as voicemail, text messaging, call recording, etc., and because they are linked to an email account, there are more bits of information that Google is able to collect whenever you use their services. Google will turn this information over to authorities if they think they are under legal or regulatory obligation to do so.
Another unwanted but expected intrusion is in the inability to wipe your data history. Although you can delete data from your active accounts, your data is likely to be backed up somewhere permanently. Unlike data stored on your own hard drive, which might also be “deleted” but still accessible, you have no access to Google's data storage to truly wipe out any record of the data like you could on your own hard drive.
The ugliest part is the increased data mining and profiling power that Google will have. This is increasingly more commonplace and difficult to avoid in any area of life. With the power over so many aspects of a user's communications, if one isn't careful, a very detailed profile of you can be built that identifies you, even if you never use your real name in any form in your communications. One must be very conscious to avoid this kind of intrusion.
Using Google Voice poses similar threats to privacy that are posed by any other service providers offering the same services, only with greater synergy. When dealing with any third party, there is always a risk that they might reveal what you have communicated to them.
Google Voice, however offers some interesting and creative opportunities to carefully craft an identity, or even several identities, much in the same way using anonymous web surfing tools can do. To do this you must selectively control every aspect of your use of Google Voice, including email usage, what computer is used to access accounts, what phone is used for forwarding and many other things that are discussed in the book How To Vanish.
Also, be sure to use other good privacy habits in conjunction with all of these services to maximize the privacy benefits. If you assume that all communications are being monitored, which in some way they probably are, you might be able to avoid serious problems.
The benefit that can be derived will vary depending on each individual's circumstances and the hassle of avoiding the pitfalls may be significant, but it may be a good opportunity to add layers of privacy to your life. Any questions about how to use Google Voice in accordance with the law should be directed to an attorney.