First, you need to understand the difference between Distinctive Ring (DR)
and Call Discrimination (CD). DR is a service you buy from your telephone
company. It assigns multiple numbers to your telephone line, and each
number rings with a different ring pattern (e.g., my main number rings with
one long ring, my first DR number rings as two short rings, and my second
DR number rings as one short ring, one long ring, and one short ring).
Getting DR is usually very easy: just call your telco and order it.

CD is the process of routing different types of call (voice, fax, data) to
the software that handles that type of call, and this is what most people 
are interested in; DR is just a convenient, but not the only, way to get CD. 
Another (clunky) way to handle CD is with Microsoft's Operator Agent, which
is a small program that voice prompts the caller to press a key on his
telephone keypad to indicate the type of call, and routes the call to the 
proper TAPI application based on the caller's response.

In order to enable CD with DR (CD/DR) under Windows, you need a modem that 
supports DR, a modem driver (INF file) that allows you to enable DR, and 
software that supports CD, either natively or with help from the Windows 
Telephony API (TAPI). If you have a voice capable modem, it almost certainly 
supports DR; some non-voice modems also support DR, especially the "soft" 
or "Win Modems". You can find out whether your modem supports DR by connecting 
to its COM port via HyperTerminal and issuing the appropriate AT command; if 
it responds with an "OK", it does, otherwise it does not. The AT command to 
enable DR depends on the modem's chipset: for the 3Com/USR/TI chipset it is 
"ATS41=1", for the Rockwell/Conexant chipset, it is "AT-SDR=7", and for the 
Lucent/Agere chipset it is "AT+VDR=1,0".

Each chipset handles DR differently.
3Com and Rockwell report ring codes ("RING A", "RING B", "RING C" for 3Com; 
"RING 1", "RING 2", "RING 3" for Rockwell), while Lucent reports the actual 
ring cadence (the duration of the ringing and the silent periods) with 
DROF/DRON messages. For example, 3Com reports two short rings as "RING B", 
Rockwell as "RING 2", and Lucent as "DRON=5", "DROF=11", "DRON=5", "DROF=34" 
(for every "ring" made up of two short rings). The 3Com and Rockwell reporting 
methods are very easy for software developers to implement, and are the ones 
required by most applications that natively support CD/DR. However, it is 
limited, and if the ring patterns provided by your telco don't match the ones 
recognized by your modem (usually two short rings for "RING 2"/"RING B", three 
short rings for "RING 3"/"RING C"), you are out of luck.

The Lucent method is much more powerful, as it allows you to select from many 
more ring patterns (and it complies with the ITU-T V.253 standard). However, 
as far as I know, it is only supported by certain versions of TAPI. I'm 
certain it is supported by Windows 98 SE, and I'm fairly certain it is 
supported by Windows 95 (OSR2), 98, and Me. If your version of TAPI does 
support it, you will be able to select ring patterns from Morse code like 
symbols (for example, ".." for two short rings, ".-." for one short ring, one 
long ring, one short ring) on the "Distinctive Ring" tab of your modem's 
properties page and assign them to different call types.