This was a post that I found in the modemhelp.net forums at:
[link]

It worked for me, of course if you break your computer by editing the registry, Storm’s Eye and it’s employees and founders cannot be help responsible.

PostPosted: 02-13-2003 07:00 PM Post subject: Workaround on Windows 2000
My need was to have my computer answer only on a particular distinctive ring pattern, so it could receive faxes without interfering with my voice calls.

Here’s how I got around it for my “U.S. Robotics 56K Fax Win Int” (part number 3CP5699A, also known as a WinModem), on Windows 2000:

(1) Turn on distinctive ring recognition in the modem:

Under Control Panel / Phone and Modem Options / Modems / Properties / Advanced, set “Extra initialization commands” to “S41=1” (without the quotes, and you don’t need the “AT” prefix).

This command varies from modem to modem; to find the right one for you, check your registry (which is populated from your modem’s .inf file when it’s installed) or your modem’s technical documentation. Mine was in the registry under HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Class\{}\0000\EnableDistinctiveRing

Instead of responding with “RING” any time the phone rings, this change makes my modem respond with “RING A” for my first phone number and “RING B” for my second number (and presumably C and D for third and fourth numbers).

(2) Tell the Unimodem driver to answer on “RING B”:

There is a “Responses” key in the registry that tells the Unimodem driver how to interpret responses from the modem. Mine was at HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Unimodem\DeviceSpecific\U.S. Robotics 56K Fax Win Int::U.S. Robotics Corporation::U.S. Robotics Corporation\Responses

(For information about the meaning of the binary data for each Responses entry, see the following URL:)
[link]

Mine already had entries for “RING A” and B and C, but all three had a response state (the first byte in the binary data) that means a distinctive ring is coming in, and the Unimodem driver for Windows 2000 doesn’t understand that. To keep them from matching, delete or rename these entries. I renamed mine and just prepended an “x” so they wouldn’t match.

Mine also had an entry for “RING”, and it had a response state (0Cool that tells the Unimodem driver the phone is ringing. I simply renamed this entry to “RING B”.

After rebooting so the changes could take effect, my computer now ignores calls on the first number and correctly answers calls on the second number. This works because the Unimodem has no responses that match “RING A” and has one response for “RING B” whose response state tells Unimodem a call is coming in.

This works for me. Your mileage may vary.

P.S. If you have the same modem model, you can find a mostly-complete AT command reference in \winnt\help\gvadwmdm.hlp, and the .inf file is in \winnt\inf\mdm3cpci.inf. Something really cool about this modem (and probably other WinModems, SoftModems, etc.): when you give the modem the AT$ command to get help, instead of giving brief text-only help, it pops up the Windows help file!