Hey Rainer,
to give you a little bit more background:
IPFire has one global repository for add-ons. There are some limitations to this, but all versions share the same repository. With the last couple of updates, we have phased out some packages and they have been removed from this repository making them unavailable for all version of IPFire.
I understand your goal, but you really should not run outdated software. We support IPFire in the latest version (and for obvious reasons the previous one), but that is it. Older versions have bugs, security vulnerabilities and many other problems that are fixed in newer versions. Therefore you should always run the latest version or you are seriously compromising the security of your installation.
To sum it up: We do not have the packages for Asterisk and the other add-ons any more. They are gone because they are known to be broken. If you really really must run a VoIP on your firewall, you can install a virtual machine with a distribution that maintains Asterisk, or install that system somewhere else on your network.
We aim to make IPFire as secure as we can, and unfortunately that means that some things have to be dropped. I am sorry if this is causing some extra work for you now, but it is the best thing to do.