Well, be responsible for troubleshooting any issues connecting to the network and importantly performing upgrades say from Debian 8 to 9. If you stick with the 3CX version of Debian - which is Debian but optimized for 3CX, not only will you reduce memory usage considerably but you will also be able to do version upgrades and security updates automatically from the 3CX management console or on auto pilot. Going with your own version of Debian is possible but requires you to do these tasks yourself and you must be comfortable and experienced in Debian Linux to do them. Most people are not.
For example, SP3 will include the option to upgrade users running Debian 8 (15, 15.5 SP1) automatically to Debian 9 with the click of a button (providing no additional packages have been installed)
We therefore recommend using the 3CX iso and dedicate it to 3CX.