When you are working with Quintum only (both ends) and you have that trouble, the issue is the “fast start feature” The fast start aparently generates a fake ringing on the ORIGINATING gateway based on messages coming trough the call setup messages.
Then, when the remote gateway switch the audio “on” , the fake ringing tone goes away and gets replaced by real audio from the remote end.
It is noticible in Quintum when you do not program the ringing tones properly, because you get a diferent ringing tone on each step.
I will play with the fast start feature on both ends, the softswitch and if posible on the ATA too.
Good luck