I agree with previous posters. Grandcentral, then when it became Googlevoice has been free for many years. It may or may not remain free in future years. Take advantage of if while you can.
As for Voip devices, most use company servers to allow you to call between like devices or to use the internet to call people with POTS (plain old telephone services). If the company goes out of business, the calls may well end and you are left with 1/2 a set of bookends.
The Obi device has a couple of things going for it. Yes, it also uses Obi servers to talk between units for free. If Obi goes out of business, those calls would likely stop. Obi also is one if not THE only device that can use GoogleVoices service. Although GoogleVoice allows free calls to the US and Canada, it also allows paid calls to international numbers. Google may or maynot continue the free calling feature; who knows. Still, the paid international calls might continue.
The Obi also allows you to configure many other VOIP services, in some cases 4 or more. This allows you to continue to use the device even if Obi AND Googlevoice stop providing services. Thus, you are assured that you can still use your device for a long time. That is a plus of the Obis over other Voip devices.
Because these devices have the ability to access those 3 types of services, they are more complicated, yet more versatile. With the competitions OOMA, you plug it in and you can make calls. At least with these Obi devices you have routing and other items not available with other devices.
I think they are good investment whether Google continues to offer free calls or not.
