Cisco IP Phones 7940 and 7960 are configured with two lines. When the user is engaged on the first line and a call comes on the second line, the caller ID for the second call is not displayed. The second call is indicated by a single tone and a small indication on the line.
This issue is described in these Cisco bug IDs:
CSCdw86063: Caller ID in the second line is not displayed
CSCdw26479: Calling Line ID (CLID) is not showing up on 794x/6x when Call Forward Display (CFWD) to the same device
CSCdu37305: 7940/60 - No caller ID and notification in the second line
The resolution is available in these Cisco CallManager releases: