I understand that leased line access offers the best voice quality for my Quintum D3000. How appreciable is the drop in the voice quality if I were to use broadband? What if the broadband has fixed ip address and 2 Mbps bandwidth up and down?
Broadband or leased lines will be fine as long as you have enough bandwith on both directions.
That is important. Most broadband providers give you a lot of bandwith on the net to customer side, but not much from customer to the net.
As long as you have 2 mb BOTH ways all the time (use the speed tool at DSLreports.com), you will be fine for a lot of VOIP channels.
Now, this is the first time that I hear of broadband that good. You could not get that on the U.S. for the normal $40- $60 dollars a month.
If the ping time on the broadband are good (bellow 300 ms to the other destination,constant or fluctuating no more that +- 150 ms, you will have first class service,doesn matter what the medium to transport it is.