Website – https://www.asterisk.org/
Asterisk is a software implementation of a private branch exchange (PBX). In conjunction with suitable telephony hardware interfaces and network applications, Asterisk is used to establish and control telephone calls between telecommunication endpoints, such as customary telephone sets, destinations on the public switched telephone network (PSTN), and devices or services on voice over Internet Protocol (VoIP) networks. Its name comes from the asterisk (*) symbol for a signal used in dual-tone multi-frequency (DTMF) dialing.
Website – https://freeswitch.com/
FreeSWITCH is a free and open-source application server for real-time communication, WebRTC, telecommunications, video and Voice over Internet Protocol (VoIP). Multiplatform, it runs on Linux, Windows, macOS and FreeBSD. It is used to build PBX systems, IVR services, videoconferencing with chat and screen sharing, wholesale least-cost routing, Session Border Controller (SBC) and embedded communication appliances. It has full support for encryption, ZRTP, DTLS, SIPS. It can act as a gateway between PSTN, SIP, WebRTC, and many other communication protocols. Its core library, libfreeswitch, can be embedded into other projects. It is licensed under the Mozilla Public License (MPL), a free software license.
Website – https://www.3cx.com
3CX is an open standards PBX that works with popular SIP trunk providers and IP phones to allow you to setup a fully functional IP PBX with a complete set of unified communications features. 3CX is well known for its easy installation and management; setup is concluded within minutes and it’s available on premise or in the cloud.
Zoiper (formerly idefisk) is a softphone that supports IAX and SIP protocols. Proprietary. There are paid and free versions. There are versions for Linux, Mac OS X and Windows.
Zoiper is compatible with SkyPBX platform, examples of settings: