Is this really needed? The Open Line feature doesn't really do anything in VoIP. The call does not start when you begin dialing or take the phone off hook. It starts when you hit SEND. All instructions are then sent to the Server for processing.
This fake dial tone does nothing on the Mac.
@rjm2884
Craig is right - the dialtone (together with touchtones, busy tone and even ringtone) is just a simple audio feedback to the user. Part of the call are the ringback and in-band DTMFs.