Since you need to keep the existing multiline phone working, and use the voip "phone" when the operator is away from the station, test using a single line phone plugged into the Aux port on the bottom of the phone. If the single line phone is still able to receive the calls from the doors, and can flash and dial 9 to bring in the outside dial tone to call a supervisor, flashing again to conference the calls, you could just use the Aux port as the "dial-tone" to the voip system.