Carrier Detect - 3Com 56K Command Reference Manual

Carrier Detect

Carrier means there is communication with the device on the other end
of the connection. Under normal conditions, the Business Modem sends
a Carrier Detect (CD) signal in response to establishing a connection with
another modem. You can use the AT&Cn command to control how the
Business Modem sends the CD signal.
.
For your modem to
Have CD always ON.
Send CD normally (the Business Modem sends a CD signal
when it connects with another device, and drops the CD signal
when it disconnects).
Carrier Detect 7-3
Command
AT&C0
AT&C1