CentOS (Community Enterprise Operating System) is a Linux distribution that provides a free, enterprise-class, computing platform compatible with Red Hat Enterprise Linux (RHEL).

As of August 2018, 20.6% of the web servers in the world, run on CentOS, according to Wikipedia.

Centos is released under the GNU General Public License.

Asterisk is an open source telephony software for making, receiving, and processing of phone calls. It is in essence a software-based PBX.

The project was started by Mark Spencer in 1999 and is distributed under the GNU General Public License version 2.

Asterisk supports many VoIP protocols, as well as both analog and digital connectivity to the traditional telephone network

Apache is a free and open-source web server software, released under the terms of Apache License 2.0.

Apache is the most widely used web server software in the world. It serves more than 100 million websites as of August 2018, an estimated 39% of all active websites.

A2Billing is a VoIP Billing Software licensed under the AGPL3.

Combined with Asterisk, A2Billing is a class 4 and class 5 telecom switch and billing system, capable of providing and billing telecom products and services such as calling card products, residential and wholesale VoIP termination, DID resale and callback services.

A2Billing provides eal time call rating, billing, auditing, and reports.

MariaDB is a high performance database server. It is a fork of the MySQL relational database management system. It aims to be a backward compatible, drop-in' replacement of MySQL.

MariaDB Server is one of the most popular database servers in the world. Its development is led by some of the original developers of MySQL.

MariaDB is distributed under the GNU General Public Licence and is comitted to stay open source.

Osmocom's OpenBSC is a project that aims to develop an all-in-one free software implementation of GSM protocol stack and elements.

OpenBSC implements several MSC (mobile switching station) components, including the A-bis protocol, the protocol between the BTS (base transceiver station) and the BSC (base station controller), AUC (authentication center), HLR (home location register), VLR (visitor location register) both using SQL tables, and a SMS Switching Center.

OpenBSC uses the GNU General Public license.

B.A.T.M.A.N.

The Better Approach To Mobile Adhoc Networking (B.A.T.M.A.N.) is a routing protocol for multi-hop mobile networks. B.A.T.M.A.N. is licensed using the GPL license.

B.A.T.M.A.N. eliminates the need to spread information concerning network changes and best route to every node in the network and creates a network of collective intelligence. Its foundation is network decentralization. No single node has all the data. Routes are rather created dynamically.

AvantFAX® is a software made for managing faxes on HylaFAX™ fax servers. It's a multi-user web application that allows users to view, send, download, email, categorize, and archive faxes online. ng all sent and received faxes.

AvantFAX is open source and is released under the GNU General Public License.

FreePBX is a web-based open-source graphical user interface that manages Asterisk.

FreePBX is licensed under the GNU General Public License version 3 (GPLv3). It has been bundled with Asterisk in the FreePBX Distro, which is a proprietary, independently maintained Linux system derived from the source code of the CentOS Linux distribution.

Oracle VM VirtualBox is a software virtualization package from Oracle that allows to host multiple operating systems on a single physical machine.

Oracle VM VirtualBox core package is released under the GNU General Public License version 2.

Separate VirtualBox Oracle VM VirtualBox extension packs that provide support for USB 2.0 and 3.0, Remote Desktop Protocol, disk encryption, NVMe and Preboot Execution Environment, are distributed under the Personal Use and Evaluation License.

HylaFAX is a fax server initially written by Sam Leffler, while working at Silicon Graphics (SGI) in June 1991.

Hylafax can run on both Unix and Linux computer systems. It can combine a large number of modems and handle very high volumes of faxes.

Hylafax is free, open-source, and is distributed under a BSD license.

Webmin is a web-based system administration tool for Unix and Linux servers.

Webmin lets you perform tasks such as: backups, passwords, files management, logs, package updates, web server configuration, database administration, user email, SSH login, networking and firewall, VPN, printer administration, cluster management, and more.

Webmin is free, open-source, and is distributed under a BSD license.