Your needs are pretty basic, as far as being met by the various SIP VoIP carriers. The market is mature, as far as capabilities and call quality goes. The very lowest-costs providers may use the cheapest VoIP carrier routes, resulting in variable call quality. The better providers strive to select and use better quality routes.
Google Voice is fine for personal (or even simple SOHO) use, but over on the Google Voice help forum, where I hang out to help, we have to rescue users daily who've put their business numbers at risk and screwed something up, thus losing control of their numbers, and losing revenue or customer goodwill. There is no direct customer support for GV.
The service provider differences that may matter to you are mostly in ease of setup and making changes, and the quality and availability of customer support. The higher-priced service providers typically have invested in better tools to enable customers to manage their numbers and customize call handling, and will have responsive customer "how-to" support. The folks who hang out in online forums like this one tend to prefer do-it-yourself solutions, which can require time dedicated to learning how to configure devices and navigate the service provider's settings. People who want to focus mainly on other things tend to choose service providers that have more of a hands-off setup.
I'd suggest starting by looking at a couple of the service providers websites. Can you make sense of it? Can you see yourself using their portal to add services, set up your OBi and set up your hunt group, voicemail, etc? If so, sign up for a number and give it a try. I like Callcentric's "sweet spot" of features, price and ease of use with OBi products, but Phonepower is another good choice for business. Services like Ringcentral and Nextiva are higher-priced, but they may be a better fit for larger business' needs.
voip.ms is a good choice too, but their portal is one step down in terms of ease of understanding for newbies.
On the OBiTALK web portal, when you click on a service provider slot (SP1--SP4) for your OBi device, it will take you to a screen where you select the VoIP service provider to use. If you scroll down to the bottom of the page, and click "OBiTALK Compatible Service Providers", you'll see a list of clickable links, which will set up that SPx with pre-configured parameters for the selected company.
If you find a provider you like, you can generally port your now-GV numbers in for free, or for a fee.
You can configure as many OBi devices as you wish to the service, and you may want to consider using Obihai's line of IP phones, too.
