Yealink SIP-T21 - Manual (Page 99)

Basic Call Features
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2. Select Enabled from the pull-down list of Directed Call Pickup.
3. Click Confirm to accept the change.
To configure the directed call pickup code on a global basis via web user interface:
1. Click on Features->Call Pickup.
2. Enter the directed call pickup code in the Directed Call Pickup Code field.
3. Click Confirm to accept the change.
To configure the directed call pickup code on a per-line basis via web user interface:
1. Click on Account.
2. Select the desired account from the pull-down list of Account.
3. Click on Advanced.