With just one OBi110, a GV account and a landline, you can set up the system to allow some concurrent calls. For example, the landline is connected to both the Line port of the OBi and phone A. Phone B is connected to the Phone port of the OBi. GV is set to both the landline and Google Chat.
When a GV call comes in, both A and B will ring and you can answer from either. If you answer from B, a second call to either number will still ring A (and give call waiting to B). If you answer from A, the landline is in use and a call to the landline will get a busy; a call to GV will ring B.
If phone A is in use, you can make a call from B (on GV only).
If phone B is in use, you can make a landline call from A. You could also make a GV call from A, by triggering it from your computer, using the OBi callback feature, or dialing through GV. This last method would be charged as a local call on the landline.
If you have a smartphone, the OBiON app can be used as an additional calling path. You can connect via Wi-Fi where available, or over 3G. The latter uses cellular data and quality is often poor, especially when in motion.