For any kind of SIP encryption, you need a Security Certificate issued ("signed") by a recognized Certification Authority, that takes on herself the responsibility to assert that you are you and not a cat.

You can cheat at this by crafting "self-signed" certificates, using software that simulate a certification authority (eg: you are your own Certification Authority). Don't. Read the section "Certificates" in chapter "WebRTC, SIP and Verto".

Certificates have nothing to do with using SSL as encryption method, "SSL Certificate" is just the old way to call a security certificate (because was then used by SSL, but it can be used by TLS too, no problem, is always the same certificate). Also if you disable SSL, you still need the same certificates for TLS, DTLS, and all other kind of encryptions.