Yealink SIP-T54S Administrator's Manual page 451

Multiple numbers are separated by commas.
Click Confirm to accept the change.
4.
To specify the return code and the reason when DND is activated via web user interface:
Click on Features->General Information.
1.
Select the desired value from the pull-down list of Return Code When DND.
2.
Click Confirm to accept the change.
3.
To configure a DND key via phone user interface:
Press Menu->Features->DSS Keys.
1.
Select the desired DSS key.
2.
Configuring Basic Features
425