GeoTrust Rapid SSL Wildcard Cert

[Karl_Yealink]

When you try to auto provision.
The server will send message to phone. If the server certificate don't include the Yealink 30 bulit-in certificate.
The phone won't accept the message from server, so you need to add the server certificate in the phone side or disable the Only Accept Trusted Certificates function. For this, you need to manual every phones.

Also the phone will send message to server side, if the Yealink CA don't include in server side, the server can't accept the phones' message.

So pleaes check the server certificate whether in the 30 bulit-in certificate or not. And also add Yealink CA to server side.

And I don't know the server certificate of your side, so you need to contact with your server provider for help.
I think you can get the certificate from them, then add the certificate in phone side.