When you install 3CX on debian it will install the latest release version. In some cases you may need to install an earlier release.
Normally we'd install 3CX with
sudo apt-get install 3cxpbx
However if we need to install a specific previous version we can issue the command
sudo apt-get install 3cxpbx=15.5.8801
In this instance 3CX version 15.5.8801 will be installed.