This is due to the fact that the intermediate certificates from your certificate CA are not included in your certificate chain for your custom FQDN certificate.
Please make sure that all the intermediate and root certificates from your provider are included in the certificate chain.
* For Linux:
1/ Connect with a file transfer client such as FileZilla using SSH root credentials to the phone system host.
2/ Go to folder /var/lib/3cxpbx/Bin/nginx/conf/Instance1
3/ Open the YOURFQDN-crt.pem in notepad ++
4/Insert the intermediate certificates and root certificate for the CA one by one separate with the begin and end certificate as follows (Make sure that your certificate for the FQDN is at the top)
-----BEGIN CERTIFICATE-----
Content
-----END CERTIFICATE-----
-----BEGIN CERTIFICATE-----
certificate content
-----END CERTIFICATE-----
Do this for all the intermediates and root certificate
4/ Save the certificate with the same name and re-uploaded it to the location overwriting the existing one.
5/ Connect with an SSH client such as PuTTY using SSH root credentials to the phone system host.
6/ Run commands:
cd /var/lib/3cxpbx/Bin/nginx/conf/Instance1
chown phonesystem: phonesystem YOURFQDN-crt.pem (enter the FQDN in place of the YOURFQDN)
Note: No space between the phonesystem: and phomesystem, here i have to put one as it was changing it to an emoji by default
7/ Then issue the following command: service nginx restart