Obi Integrates with google voice via the google CHAT interface. When you get your GV number you just follow the obi instructions for google which will have you check the box to enable google voice to send calls through google chat.
You don't need any other number if you don't want it. Though if this is a primary home number, you typically will want 911 service, which google voice doesn't provide. To get E911 you will need to sign up for services from some other provider, some may require you to purchase a DID number, others do not.