Install old Linux version of 3CX

apt-get install dphys-swapfile
wget -O- http://downloads-global.3cx.com/downloads/3cxpbx/public.key | apt-key add –
echo “deb http://downloads-global.3cx.com/downloads/debian stretch main” | tee /etc/apt/sources.list.d/3cxpbx.list
apt-get update
apt-get install libcurl3=7.38.0-4+deb8u5
apt-get install 3cxpbx=15.5.20000.6