Hi:
Well, you'll probably get a bunch of opinions in answer to your question.
Here's my perspective, as a long time "Top Contributor" on the Google Voice help forum: Google Voice is great for individual/personal use. There is no specific prohibition against using Google Voice for business purposes. However, we strongly advise against it. GV has no direct customer support, no guaranteed service level (it is not covered by the Google Apps service level agreement), and it does occasionally have difficult-to-diagnose problems. When (not if) that happens, any small savings in calling costs you may have incurred will be quickly wiped out by loss of business or your business' reputation, when your customers can't reach you.
So: use GV for business purposes if you wish, duly advised of risk. Otherwise, there are plenty of good independent SIP VoIP providers from which to choose, including PhonePower. You can easily port your number to them, or to some other provider of your choice, and add it as another SP configuration to your OBi.
