Daley,
ISDN channels are 64kbps each,
or 128kbps when combined into one connection.
(you do always have 2 base channels per "wire-pair")
So the whole thing is a matter of your modem using
both channels at the same time...
No contribs nothing ...
...maybe just a modem string change.
For starters: What ISDN modem do you use ? Where ?
Regards
Reinhold