3Com 56K manual Carrier Detect, Business Modem sends the CD signal, AT&C0, AT&C1

Carrier Detect 7-3

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	Command
Have CD always ON.	AT&C0
Send CD normally (the Business Modem sends a CD signal	AT&C1
when it connects with another device, and drops the CD signal
when it disconnects).