Yealink SIP-T21 - Manual (Page 131)

Troubleshooting
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4. Select the desired value from the pull-down list of RTP Encryption (SRTP).
5. Click Confirm to accept the change.
Note
How to reboot the phone?
To reboot the phone via web user interface:
1. Click on Settings->Upgrade.
SRTP is not available on all servers. Contact your system administrator for more
information.
SRTP is configurable via web user interface only.