Dont ask how I know, I shouldnt be telling you this without you ringing us...
Ill use win98 as an example to set this up...
Do NOT forget it is 2x local call rates for 128k connections!
Double click on make new connection in Dialup Networking
type a name that will tell you it is 128k
next
type 08457640000 as the number to ring
next
finish
right click on new dun icon in Dial up networking
properties
servertypes
untick all apart from tcp/ip
click multilink
select use additional devices
add
set device to second isdn channel
set phone number in this section to 08457128000
click ok etc, and thats it!
Notice the numbers there... 7640000 for 64k, and 7128000 for 128k connection! The initial number MUST be the 64 number, and the additional should be 128 number... And its connect as normal.