Yealink SIP-T54S Administrator's Manual page 449

(Optional.) Enter the DND off code in the Off Code field.
3)
If you mark the Custom radio box:
b)
Select the desired account from the pull-down list of Account.
1)
Mark the desired radio box in the DND Status field.
2)
(Optional.) Enter the DND on code in the On Code field.
3)
Configuring Basic Features
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