It is possible, however, it may not be the correct thing or work the way you desire. You can setup the pstntg to provide a dialtone so that incoming callers can call in on the FXO, get a dialtone and then dial out to IP. Additionally, you can add a pincode so that when they dial in they have to enter a code before they get dialtone.
If you would like more information, please email me at mike_voip@hotmail.com.
Mike