I am sure that 10 people will offer 7 different solutions, one of which will include 54 lines of dial plan code :-P
But, keeping it simple, you may only need one OBi. Do you already have telephone wire running into the room with the hardwired phone? If so, depending on whether you are also using that wiring for land line (POTS) telephones, you can just disconnect one pair or all pairs of that wire from your outside telephone company demarcation block, and then plug the OBi's phone port into your home wiring. Then plug as many phones (cordless base stations or corded phones) as you like into the jacks around the house.
Otherwise, using multiple OBis, port your Google Voice number over to a quality SIP VoIP carrier. I use Callcentric. With Callcentric, you can define multiple extensions, and then set up call-handling rules to forward inbound calls to any or all of those extensions. Each extension is a separate SIP registration, so it just looks to the OBis like you have multiple service providers. For example, if your primary Callcentric DID's ID is 17771234567, then extension 101 is 17771234567101. You'd configure that SIP credential on whichever OBi SP port you wish.
I have this very thing working at home. I have one OBi 110 with two different Callcentric DID numbers, one configured on each of the two ports on the 110. I also have a Gigaset IP cordless phone system with the same two numbers configured as two different extensions. Finally, I have an antique phone configured with a Grandstream HT701 ATA, with yet another extension.
When an inbound call to one of my two Callcentric DIDs comes in, it rings the hardwired phones plugged into the OBi, and the Gigaset cordless phones, and (for just one of the two DIDs), rings the Grandstream-attached phone.
