If someone wanted to maintain the PKGBUILD for this project, it’d be trivial to include a patch that removes the code he added trying to make it not build.

Or, to make sure to not be in breach of the no-derivatives part of his lisence, just reimplement it and ship with a patch that fixes his “blocker”.

I wonder if he received permission from all the other contributors to change the license of their contributions.

[…] nobody uses SIP…

Say what?

In my part of the world signaling for literally every phone call, be it mobile or fixed, traverses networks and operators using SIP.