Yealink SIP-T20 - Manual (Page 49)

Customizing Your Phone
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You can use two headsets when enabling dual headset. To use this feature, you must
physically connect headsets to the headset jack and handset jack respectively. Once
the phone connects to a call, the headset connected to the headset jack will have
full-duplex capabilities, while the one connected to the handset jack will only be able to
listen.
To enable Dual Headset via web user interface:
1. Click on Features->General Information.
2. Select Enabled from the pull-down list of Dual-Headset.
3. Click Confirm to accept the change.
Note
Two types of DSS keys: Line Keys and Programmable Keys. The details will be introduced
in the following. SIP-T20P/SIP-T20 IP phone supports 2 line keys.
Dual headset is configurable via web user interface only.