Hi Asim,
The 69xx series are a tad different than some/most Cisco IP phones
Configuring the Phone to Support Call Waiting
The Cisco Unified IP Phone 6921, 6941, and 6961 support one call per line. Cisco Unified Communications Manager sets the Maximum Number of Calls (MNC) per line, and Busy Trigger (BT) per line attributes to 2 and 1 respectively and these fields can not be changed, As a result, the line appears to be busy to a second incoming call when there is an active call present on the line. Cisco Unified Communications Manager forwards the second incoming call based on the Call Forward Busy settings for the line. The user, who is on a call will not be notified of an incoming call. Without further configurations, the phone does not support call waiting. However, you can configure the phone to support call waiting and receive the call by configuring an additional line on an unassigned programmable button. When the user receives an incoming call while connected on a call, the phone plays the call waiting tone (single beep), displays a caller ID notification and rolls the incoming call over to the other line.
To configure call waiting on the Cisco Unified IP Phone 6921, 6941, and 6961:
1. 
On an unassigned programmable button on the phone, configure an additional line by either assigning the same directory number on a different partition or by assigned a different directory number.
a. When you configure the second line by assigning the same directory number on more than one line, the DN on the second line must be on a different partition. Each time that you assign the same DN to another line, you must create a unique partition and assign the directory to that partition.
b. When you configure the second line by assigning a different directory number, you must use a previously unassigned and unlisted number.
The second line is an intermediary between the original line (your listed directory number) and your call busy destination, such as your voice messaging system.
2. On the original line, configure the destination for the Call Forward Busy Internal and Call Forward Busy External settings to your second line and the No Answer Ring Duration to a minimum of 10 seconds.
3. On the second line, configure the standard Call Forward All settings that you would normally use for the original line and set the No Answer Ring Duration to a minimum of 10 seconds.
Note You will need to assign a programmable button for each listed directory number.
Cheers!
Rob
