1. Log into your 3CX server via SSH
  2. Open /etc/apt/sources.list with nano
  3. Add the following

deb http://deb.debian.org/debian/ buster main

deb-src http://deb.debian.org/debian/ buster mai
  1. Save the sourses list
  2. run apt-get install qemu-guest-agent
  3. Edit your sourses list again to remove the above repo