Voice over IP (VoIP) software enables telephone-like voice conversations across IP based networks. A VoIP phone service is often cheaper than a traditional Public Switched Telephone Network (PSTN) phone service and removes geographic restrictions to telephone numbers.

SIP is the most popular VoIP protocol. This protocol enables two or more people to make phone calls to each other using the Internet to carry the call. SIP to SIP calls on a broadband internet connection are high quality, always free regardless of distance, and offer additional functionality such as free voicemail to email and phone numbers, caller ID, 3-way conference, speed dialing, call forwarding, simultaneous ring, call waiting, call return, caller ID block, and anonymous call rejection.

Softphones are user based clients for making and receiving voice and video communication over the IP network, usually permitting integration with USB and IP Phones instead of a computer’s microphone and speakers (or headset). This article selects the best softphones available for Linux.

We have included Skype in the list. Whilst the Skype client is available for no-charge, it runs on a closed proprietary network, and is therefore not ‘free’ software. However, given the importance of Skype in promoting VoIP, we believe it warrants a place.

For enterprise or business markets, VoIP enables the enterprise to manage a single network (the IP network) instead of separate voice and data networks, while enabling advanced and flexible capabilities to the end user. With the credit crunch still to take full effect, it’s even more important for small businesses to minimise their costs. One way to reduce expenditure is to switch from a normal telephone contract to using VoIP. Calls made using VoIP work and sound like normal telephone calls, but cost significantly less. Public Branch Exchange (PBX) is a private telephone exchange which routes calls internally and provides call queuing, hunt groups, conference calling, voicemail and more. The PBX enables many phones to use a single VoIP connection.

So, let’s explore the eleven VoIP software at hand. For each application we have compiled its own portal page, a full description with an in-depth analysis of its features, screenshots (where relevant), together with links to relevant resources and reviews. Start making free calls today.

VoIP Softphone clients
QuteCom VoIP softphone originally known as WengoPhone
Ekiga VoIP and video conferencing application for GNOME
Linphone SIP video-phone
Twinkle Qt-based VoIP phone
Kiax Simple user interface for making VoIP calls to Asterisk PBX
Skype Skype peer-to-peer Voice over Internet Protocol (VoIP) software
KPhone Supports Presence and Instant Messaging
Server Software
Asterisk Open Source VoIP PBX
FreeSWITCH Telephony platform to facilitate the creation of voice and chat driven products
GNU Gatekeeper VOIP gatekeeper for H.323
sipXecs IP based communications system (IP PBX)