Turn “faststart” on. It will give the caller a fake ringback when the other gateway is dialing the number.
Two troubles: If the destination number is busy, it will sound like is ringing and then switch to busy tone (the real one from the phone company)
Second one: At least with the default settings, the ringback will be “american ringback”. Some people will know that there is fake because the normal ringing in same places is diferent.
You could try with the regional settings on both ends and see if that changes anything.
If you bought the equipment from a Quintum dealer, give the a call. They are good.