Yealink SIP-T54S Administrator's Manual page 974

Administrator's Guide for SIP-T2 Series/T19(P) E2/T4 Series/T5 Series/CP860/CP920 IP Phones
Click Browse to select the certificate (*.pem, *.crt, *.cer or *.der) from your local system.
2.
Click Upload to upload the certificate.
3.
To configure the server certificates via web user interface:
Click on Security->Server Certificates.
1.
Select the desired value from the pull-down list of Device Certificates.
2.
Click Confirm to accept the change.
3.
To upload a server certificate via web user interface:
Click on Security->Server Certificates.
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