Yealink SIP-T20 - Manual (Page 89)

Basic Call Features
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5. Press or to select Enable from the Anon Rejection field.
6. (Optional.) Enter the anonymous call rejection on code in the Reject On Code field.
7. (Optional.) Enter the anonymous call rejection off code in the Reject Off Code field.
8. Press to accept the change.
Anonymous call rejection is configurable via web user interface at the path
Account->Basic.
5. Anon Rejection:
◄ Enable ►