One thing to think about though is a failback if 2FA isn't working... such as a really long recovery code... What if the only admin person's phone died and he gets a new one and his 2FA is lost? Ooops. Believe me, I've been there.No, I see little point in making it selective, the idea is to protect the PBX. Some of the previous security issues have needed gui access. I want this to be able to be applied to potentially older systems that cannot be upgraded because of heavy modification etc. In fact that is my primary need. I have older systems out there that I do not want to upgrade. Combined with the whitelist/blacklist thing I am working on that makes the sip server invisible to the net, I'm in with a fighting chance.
I just finished going through this. Dropped my phone in a lake and getting all my 2FA authenticators going again on a new phone was a major pain.One thing to think about though is a failback if 2FA isn't working... such as a really long recovery code... What if the only admin person's phone died and he gets a new one and his 2FA is lost? Ooops. Believe me, I've been there.