Certificate not trusted

[loop]

I have an OpenSIPS gateway with a bought TLS certificate (not self signed).
I am successfully able to connect T20 handsets and Windows Zoiper soft phones to the gateway. They both trust the certificate.

Unfortunately I can not get the W52P to trust the certificate. I have to set "Only Accept Trusted Certificates" to "Disabled".

I've tested the certificate by using it with Apache for a HTTPS site and all browsers from multiple operating systems see the certificate as trusted. This leads me to believe the certificate is valid.

Is there any guidance on what kinds of paid certificate the W52P is compatible with? Is this a bug with the phone or do I need to buy my TLS certificate from a particular vendor?