A dual line is configured through overlay-dn on Cisco CallManager for an IP phone. When the first line is in use and a call arrives on the second line, no audible alert is heard. Users will notice that there is an incoming call only by looking at the IP phone display.
The audible alert (beeping) was meant to be off by design (to be less annoying) to all users on the overlay-dn.
If the audible alert is indeed required for a customer, then the workaround is to use a separate button on each phone for overlay-dn and another button for each phone's actual Directory Number (DN). This way, if a user has an active call on the actual DN and receives another call, the user then hears a beep, indicating an incoming call.