Is this the tweak your talking about?
Borrowed from Axcel216
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This tweak applies also to 56K/ISDN modems connected to internal ISA (PnP)
serial I/O add-on cards equiped with faster UARTs: 16650 or 16750, designed to
transfer data between the COM port and the modem up to 921200 bps. Wow! This
makes regular ISDN look like a slow poke.
But this canNOT be usually achieved without a Registry tweak, because the
maximum transfer speed (default) allowed by Windows 9x is 115200 bps, enabled
from: Control Panel -> Modems -> Your Modem Name -> Properties -> General tab
-> Maximum speed -> 115200 -> OK -> OK.
To enable and use this higher speed, run Regedit and go to:
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Class\Modem\0000
Note that your modem might be installed under the 0001, 0002 etc keys, not
necessarily under 0000. Highlight the appropriate one. In the right hand pane
double-click the "Properties" Binary value. You'll notice that the 7th up to
the 10th digits on the 4th row (the one that begins with 0018) show C2 01 (or
84 03), which corresponds to a maximum connection speed of 115200 (or lower)
bps. Highlight and replace them with 10 0E to bump it up to 921200.
Geek speak: replace the 24th up to the 27th byte (0x18h) as shown above.
Do NOT change anything else there! Click OK when done.
For convenience you can use the REG file below to do this without having to
alter the Binary value. Just make sure the modem key matches the one in your
Registry (replace 0000 with 0001 or 0002 etc if necessary):
-----Begin cut & paste here-----
REGEDIT4
[HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Class\Modem\0000]
"Properties"=hex:80,01,00,00,ff,00,00,00,ff,00,00,00,07,00,00,00,0f,00,00,00,\
77,07,00,00,00,10,0e,00,00,fa,00,00
------End cut & paste here------
Paste these lines into Notepad (the empty line at the end is necessary!) and
save the file as 921200.REG. Then (double)-click on it in Explorer or File
Manager to merge it into your Registry.
Now if you open Control Panel -> Modems... and follow the same steps above,
you can see that 3 new settings are available: 230400, 460800 and 921600.
Reset your modem's "Maximum speed" to one of these new values (some
experimenting might be necessary depending on your hardware specs), save your
changes, and finally, connect to the Internet as usual. I'm sure you'll notice
the speed boost.
CAUTION:
If you try this on slower internal 56K modems, your connection may lock up,
and/or experience various errors while using any communications apps or the
Internet, which may also occur if your external modem doesn't have a fast CPU
(25 MHz or above). If that is the case, reset the "Maximum speed" (see above)
back to 115200.