My understanding is kamailio has nothing to do with the media, so it's an application that sits in front of FS, handling signaling.
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https://www.slideshare.net/oej/kamailio-a-quick-introduction
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If it were just the asterisk boxes (and not individual registered users), I'd just use ip auth (see permissions) and either allow_trusted or allow_address...
if (allow_trusted()) { route(PSTN); #or return; etc... };
> The next release of Kamailio will be 4.3 – a release with new module and many new functions! The GIT branch for the coming release 4.3 has been created. This branch will host the release series 4.3.x. To help the developers, please download this branch and test it before the release! > > > > To get this branch from GIT, you can use: > > > > git clone https://github.com/kamailio/kamailio.git kamailio > > cd kamailio > > git checkout -b 4.3 origin/4.3 > > > > Notes about installing Kamailio from this branch are available at: > > > > http://www.kamailio.org/wiki/install/4.3.x/git > > > > Hopefully in about two weeks or so the full release of 4.3.0 will be out. > > > > The master branch is again ready for new features, to be part of the next future release, expected to be numbered 4.4.