Fresh Debian 10 install. This method is unsupported by 3CX for some reason.
apt install gnupg
wget -O- http://downloads-global.3cx.com/downloads/3cxpbx/public.key | apt-key add -
echo "deb http://downloads-global.3cx.com/downloads/debian buster main" | tee /etc/apt/sources.list.d/3cxpbx.list
apt updatesudo apt update
sudo apt install -y net-tools dphys-swapfile
sudo apt -y install 3cxpbx