Yealink SIP-T21 - Manual (Page 87)

Basic Call Features
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forward for each or all accounts. Two call forward modes:

Phone (default): Call forward is effective for the phone system.

Custom: Call forward can be configured for each or all accounts.
To configure the call forward mode via web user interface:
1. Click on Features->Forward & DND.
2. In the Forward block, mark the desired radio box in the Mode field.
3. Click Confirm to accept the change.
Note
To enable call forward in phone mode:
1. Press Menu->Features->Call Forward.
2. Press or to select the desired forwarding type, and then press the Enter
soft key.
Call forward mode is configurable via web user interface only.