I can't make sense out of your description, so I'll just reply with how it should/does work.
If you now have a Google Voice inbound number (people can call that number, and their calls are then forwarded to your cell phone and Verizon numbers, then an OBi will simply log into the Google account associated with your current GV inbound number. OBi devices are Google Chat clients. You will be forwarding the same calls, to the same, current GV number, to Chat + cell phone + land line numbers. The inbound calls will ring on the phone attached to the OBi, and to the same two forwarding phones as before.
If you want to get rid of the Verizon land line, then you have several choices. Google Voice can host two different inbound telephone numbers on one account. All calls to either number will be forwarded to the same forwarding destinations (your OBi via Chat, and to your cell phone number). Outbound calls will use the caller ID of whichever of the two numbers you designate in GV settings as "primary", and you can swap primary vs. secondary roles at any time, for free. The limitation of this setup is that you can't differentiate separate VM greetings for each inbound number (all inbound calls, to either now-GV number, are handled the same way).
You can port that land line number into a prepaid cell phone account (use AT&T GoPhone or T-Mobile Prepaid, not some Mickey Mouse reseller). Wait a week, then, assuming it is eligible to be ported into Google Voice, you can port it into the same Google Voice account you are already using today. The cost to port a number into GV is $20. There is a second (additional) $20 fee to keep the first number which makes that number permanent.
The alternative is to port the land line number to a different/separate Google account. You can then provision the OBi with both Google accounts, one on SP1, and the other on SP2, and use SP3 for E911 service, for example. The advantage of this approach is that each Google Voice number is independent of the other, and can have separate VM greetings and separate call forwarding setup. The disadvantage of this alternative is that a cell phone number can only be a forwarding phone number on one Google Voice account. This is because of the way GV handles text message forwarding. In theory, you could change the cell phone type in GV to "home" or "work", which turns it into a pseudo-land line. In that case, it could be used as a forwarding phone number on two accounts, but text messaging via GV would be disabled to that cell phone number.
