3Com 3C871 Quick Start Instructions page 15

10 Reset the digital modem by turning the power switch off
and then on again.
If the parameters match those registered at the telephone
company ISDN switch, the D channel LED lights for a few
minutes, flashes for several seconds, and then turns off. If the
LED continues to flash or remains lit, see Chapter 3,
"Troubleshooting," in the 3Com Impact ISDN External Digital
Modem User Guide.
11 Go to the section "Placing and Receiving Calls" in
Chapter 2 of the 3Com Impact ISDN External Digital
Modem User Guide to place your first call.
Configuring the 3Com Impact Digital Modem 15