Modem problem

I’ve got a ZyXEL ISDN modem that works perfectly under
Slackware and FreeBSD.

The thing is that I can’t connect with my ISP with a linespeed (PC
<-> modem) of 115200 (it dials an negotiate… and then hang up).

If I reduce the linespeed to 57600 I can get a connection. But it’s
not a very usable connection when I use it for heavy dwl (eg. dwl
from repositories). After a while the dwl stalls but I can see that
my modem still receive, and receiving so much that the Rx-button
isn’t flashing but ON. I even get a high-pitch sound from the modem.

Perhaps it’s due to the difference in speed: PC ← 57600 → modem ←
64000 → world.

Anyone knows why connection fails at 112500 and why it’s so lousy at

And does anyone have a solution?