If you have the IVR server on the same segment that the Quintum gateway, and still have troubles,for sure is not the bandwith.
My experience with the low bandwith issue is like this:
You place a call to the Quintum, the call get answered, and when the gateway tries to give you the balance or the ammount of minutes, the file is not present.
The Quintum time out, and you get a error message.
If you call right back, now the file finished downloading and the prompt will play properly.
But if you change the ammoount of minutes or the balance on the billing, forcing the Quintum to play a diferent propmt,the trouble will show again.
After a while, all the prompts will be played at least once, and for that reason they will be on the memory of the unit.
All this could be seen on the log if you enable the event log to see the IVR function.