Arm-device could be IP PBX, very easy to install both freepbx(asterisk) and fusionpbx (freeswitch) on debian, Just follow official tutorial
512MB memory at least, 1GB recommended. it's better to install plain asterisk or freeswitch if memory less than 512MB, but need more time to configure/debug.
Fusionpbx(Freeswitch): https://www.fusionpbx.com/download.php
Install Script
Debian Install
Debian 8 is currently the recommended operating system by the FreeSWITCH developers. Start with a minimal install of Debian 8. You can download it from here. Then run the following commands as 'root'.
wget -O -
https://raw.githubusercontent.com/fusionpbx/fusionpbx-install.sh/master/debian/pre-install.sh | sh;
cd /usr/src/fusionpbx-install.sh/debian && ./install.sh
Note: If using Debian Jessie on Proxmox LXC containers please run the following BEFORE starting the FusionPBX install.
apt-get update && apt-get upgrade
apt-get install systemd
apt-get install systemd-sysv
apt-get install ca-certificates
reboot
Freepbx(asterisk):
freepbx 13 on debian 8.1:
https://wiki.freepbx.org/display/FOP/Installing+FreePBX+13+on+Debian+8.1
freepbx 14 on debian 8.8:
https://wiki.freepbx.org/display/FOP/Installing+FreePBX+14+on+Debian+8.8