I was testing some things with Certificate Manager from another thread and I have found that a LE cert installed via Certbot on a standard FreePBX 14 / SNG7 system works just fine with all versions of Yealink phones and firmware that I had easily available for testing.
To me, this brings it back to something in the Sangoma implementation of LE is different. I mean it works fine in all the browsers, but the phone refuses to talk to it.
But a Certbot generated cert does work? What is the difference? Certbot is known, trusted, and extremely widely used. Sangoma’s implementation of ACME? Who knows? I’m not qualified to answer that. Just looking from the outside in.