I don't understand fully your question.
An OBiWiFi is used instead of a wired ethernet connection between your OBi200 and your router. It has nothing to do with how your phones connect to the OBi200.
My guess is that you want one phone connected upstairs to your existing phone service. I cannot tell if that is a conventional copper telco line or a VoIP equivalent provided from a phone socket on your ADSL or cable modem-router but it makes no difference. You want the second phone downstairs plugged into your OBi200 which is connected to the router by Wi-Fi.
If my guess is correct, then yes that will work if the path between OBi and router is not giving too much loss.
