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.