setting up a VOIP clean room phone

I have not, but I can tell you it is possible to use unsupported third party SIP phones.  Just plan ahead and expect it to take some days/weeks to get approved and enabled on your account.  I had to go through support and also have an account manager approve the request and it took about a week to get the ability to use a Grandstream ATA on a client account.  You will also be on your own to figure out how to configure the device for TLS and load certificate, etc.

These were the specifics support asked for and would be required of a third party device:

Please verify that the device(s) support:

• SIP registration
• Wildcard SSL certificate and 
• TLS1.2/SRTP security protocols (provide the list of TLS 1.2/AES-256 cipher sets for the device(s))

We would appreciate it if you shared a screenshot or the related documentation.

Good Luck.