Yealink SIP-T21 - Manual (Page 83)

Basic Call Features
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for example “2/3”, indicating that this is the second call out of three calls.
You can use DND to reject incoming calls automatically on the phone. Callers will
receive a busy message.
You can enable/disable DND for the phone system, or you can customize DND for each
or all accounts. Two DND modes:

Phone (default): DND is effective for the phone system.

Custom: DND can be configured for each or all accounts.
You can receive incoming calls from authorized numbers when DND is enabled.
To configure the DND mode via web user interface:
1. Click on Features->Forward & DND.
2. In the DND block, mark the desired radio box in the Mode field.
3. Click Confirm to accept the change.
Note
To activate DND in phone mode:
1. Press the DND soft key when the phone is idle.
DND mode is configurable via web user interface only.