Win 2k Slow RAS dial-in speed

I have setup RRAS on a win2k server with a 4 modem pci card and I have tried everything to increase the connection speeds obtained by remote clients. The maximum speed we can get is around 28kbps. I have tried different servers, diferent configs anddifferent phone lines but still no change. I have tested dial out from the actual server to an ISP using one of the modems on the PCI card and acheived connection speeds of 56kbps.
Ive read about numerous things to try to increase speed on NT4 servers but nothing much for Win2k servers.