[pptp-server] speed problems/packetloss

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dear all

I use poptop to setup a VPN between a WIn98 box and a linux server
for voice over IP calling
when a call is paced directly from the win98 box (using dialpad.com) the 
call audio is fine
when a call is placed from the NT box next to the VPN server the call is fine
but when I call is placed from the Win98 box VIA the VPN itself (56K modem 
link) the incoming audio stream is ok but outgoing isnt ....

anyone tell me why?