Let’s see if you are in that situation and what to do.
Prerequisite
A cloud instance with Debian ISO installed on it. We will be trying Debian Linux 10.
Adding the repository
Run the commands below as the root user:
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 update && apt install net-tools dphys-swapfile htop mtr screen curl dnsutils -y

Commands for installing 3CX
apt install 3cxpbx -y

Start the installation
The installation already began with the previous command apt install 3cxpbx -y. When asked, Press Tab to select the <Ok> button and press Enter to accept the 3cx license agreement.

Finish the Installation
When the installation is finished, you will see the following screen, which asks you to select the configuration option. I’d recommend typing 1 (which selects Using a Web Browser) and hitting Enter, which will show you the URL to configure 3CX.

To configure 3CX and finish, you might look into this article.
Rakib knows what he is doing…
Thanks a lot. I really appreciate it.