Your experience should weigh in, right? If Google Voice works well for you, just stick with it. It is, in fact, highly reliable, but not perfect. Many problems are caused by users themselves, who, for example, forget their user ID and password and complain about all the business calls they're losing, or other such human error. Occasionally, problems occur with calls to or from limited geographic areas, or between certain carriers.
Phonepower is a great alternative. Many of us also prefer Callcentric or
voip.ms. You can get a very inexpensive, pay-per-minute account with one of those two, and configure it as a backup on one of your OBi's other service provider slots. Keep in mind that some sort of E911 service is essential, and by adding one of the SIP VoIP providers to your OBi, you've got that covered.
