There will be some maintenance work done to deb.kamailio.org server these days in order to move to a better infrastructure, so it may not be available for a while. The DNS needs to be updated as well, this also can affect you if your router/provider does caching.
This is affecting the APT repository for Debian and Ubuntu packages.
Update Sep 28: migration has been completed, if you encounter any issue with the apt repositories, contact us.
Thanks for flying Kamailio!