Pardon me for butting in, but:
A whole thread could be devoted to selecting a VoIP service provider, independent of whether you want to use it with your OBi or not.
The questions to ask, posted previously, might make sense for a technically-inclined enthusiast, but the poster was asking about use for an alarm system and for faxes. I cringe when I see people discussing this without considering the risks.
For your alarm system, do you really want to bet your safety on the cheapest provider you can find? If you need maximum reliability, I'd argue that you should keep a traditional POTS (Plain Old Telephone Service) local telco voice line, or, change to a monitored system that uses a cellular connection. If you are willing to sacrifice some reliability for cost, then sure, use a VoIP provider, or switch to NextAlarm (which uses your broadband internet connection), but keep in mind that it won't work if your power is out or cut, or your ISP goes down. This is the main reason I have kept a "legacy POTS" line.
That said, I have used a lot of different VoIP services over the years, and I am currently using Callcentric's pay-per-minute offering for voice calling and faxing. It would be the next best choice for an alarm monitoring service connection, given the disclaimers above, and you could provision an OBi 110's phone line (FXO) port to use Callcentric (along with, or instead of, Google Voice), or buy a different Analog Telephone Adapter (ATA) to use just for that purpose.
For extra safety: if you choose this option, at least hook up all your gear (ISP modem, router, Obi) to a good UPS.
You may know this, but monitored alarm systems (should) use a specially-wired RJ-31X exclusion phone jack, such that the (telco or VoIP) phone line is connected to THIS jack first, then the alarm panel's telco line plugs into the RJ-31X, and any other phones on the premises are wired downstream from this jack. This allows the alarm panel to "seize" the line when it needs to call the monitoring center, disconnecting any other phones that might be off-hook, when it's using the line.