Google is a wonderful company, but Google Voice is not really for small businesses as its functions are simply not robust enough. Personally, I think Google Voice is good for allowing for people to not find you, which is the opposite of what small businesses want.
For example, some analyst firms offer free whitepapers written by vertical industry experts. It helps to be able to get the statistics that these firms have spent tens, sometimes hundreds, of thousands of dollars on. But often filling out these forms means that you’ll get a phone call or a series of emails asking me to subscribe to a premium service. Google Voice, in these instances, is a great way to give out a number for such marketing lists. I can check the voicemails when I want, but can also choose when and when not to have such calls go to my cell phone.