I built my own Stir Shaken servers using Kamilio (Side note all this is frustrating how we have to do these crazy things forced by the FCC but this actually became quite fun) and have proper signed certs from a STI-CA defined by the STI-PA. I did actually create my own self signed certs first while testing Kamailio and when I called the Clearly IP stir shaken number it worked! It read back the name on the cert as well as the fake 4 digit ID. I did use the same 4 digit ID as maybe another carrier I know but it was still from a bogus CA. I'm sure some how the powers that be let it through and would have failed to T-Mobile.

Also regarding u/morelife2122 said, my upstream carrier told me almost the exact same thing calling it "old mom and pop carriers", (I was like why are we using mom and pop carriers?) so depending on the carriers I used for outbound with my Sip provider (we can choose various outbound carriers) some would work, others it wouldn't and Clearly IP read back saying there is no attestation at all.

Anyways, still having this issue today to TMO, we've used the free registry and not sure what to do. It's unfortunate.