I get the same message "Host SSL Certificate is untrusted"
when I try to connect to my companies OCS under https://sip.company.com:xxx
is the domain of my company and xxx are figures (the Port I guess). This address is exactly the same as given in OCS options under My Account, Extended settings, Manual configuration, External server name or IP-Adress. Connection type is TLS. With these settings the PC client works fine over the internet.