Email from Rick Matthews:

------- From <http://www.modemsite.com/56k/dring.asp>-----------------

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).
