On a supervisor or agent ACD phone, you can use a DWC key.
when the DWC key is pressed, the Calls Waiting status for the ACD DN is displayed as follows:
aaa - bbb - ccc - dddd
Where:
aaa = the number of calls currently waiting in the queue
bbb = the number of agent positions occupied for the ACD DN
ccc = the waiting time in seconds of the oldest call in the queue
ddd = always zero (0) with Basic ACD and does not apply