Yealink SIP-T20 - Manual (Page 76)

User Guide for the SIP-T20P/SIP-T20 IP Phone
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To configure the DND authorized numbers via web user interface:
1. Click on Features->Forward & DND.
2. Select Enabled from the pull-down list of DND Emergency.
3. Enter the numbers in the DND Authorized Numbers field.
For multiple numbers, enter a comma between every two numbers.
4. Click Confirm to accept the change.
When DND is enabled on the phone, the phone can still receive incoming calls from
the numbers specified in the DND Authorized Numbers field.
Note
You can configure your phone to forward incoming calls to another party by the static
forwarding. You can also forward calls while your phone is ringing, refer to the dynamic
forwarding.
Static Forwarding
Three types of static forwarding:

Always Forward: Incoming calls are immediately forwarded.

Busy Forward: Incoming calls are immediately forwarded if the phone is busy.
DND authorized number is configurable via web user interface only.