When you have the telephone handsert connected to the Linksys SPA3102, simply dial #0 or #1 to switch between the MyNetFone network and Telstra network.

Also, to check whether the fallback feature on this device is setup correctly, follow the setup instructions, then make a few calls and login to the MyAccount Portal to check whether the calls have been billed by MyNetFone (and not your landline provider).