Your trouble is lack of bandwith.You do not have enough “pipe” for voice and data at the same time.
I own some Quintum A200 and on those there is a setting that lower the ammount of bandwith used by the computer when there is a voice call in progress, releasing all the bandwith to to the P.C. when the call is over.
The only trouble: The minimun that you could set the bandwith is 64 kb.
In my case ,I am using ADSL that is 64 kb both ways (very cheap). for that reason it does not work for me.
I read something about bandwith reservations when you use Cisco routers, but have not idea if it is related to your trouble or not.
Good luck!!!