Voice over Internet Protocol (VoIP) is a term used to describe the transmission technologies for delivery of voice communications over Internet protocol (IP) networks such as the Internet. Other terms frequently encountered and synonymous with VoIP are IP telephony, Internet telephony, voice over broadband, broadband telephony, and broadband phone.
Internet telephony refers to communications services -voice, facsimile, and/or voice-messaging applications—that are transported via the Internet, rather than the public switched telephone network (PSTN).
There are many Linux clients for voip.
* Twinkle, my choice, even not the best, easy to configure
* Ekiga, claimed to be the best. but I haven’t made it work for me. In fact, it is easy to configure. “SIP, H.323 audio and video softphone for various linux, solaris, windows, and various unix systems. Formerly GnomeMeeting”
Here is a cascade of simple steps: Start Ekiga -> Edit -> Account -> Add sip account
* Linphone audio and video SIP softphone for Linux and Windows XP * minisip cross-platform SIP softphone, Linux, Windows XP and soon Windows Mobile 2003 SE I have this one tested with my VOIP (mynetfone too), in fedora 13. works well.
* FreeSWITCH: Console client for SIP, IAX2, Woomera and Jingle/Google Talk
* MjUA: simple cross-platform SIP softphone, written in java, based on MjSip stack
* Open IP Phone Business IP Phone sdk support, ims compliant, good interoperability.
* OpenSIPStack MPL licensed SIP stack with ENUM, Presence (XMPP/SIMPLE) and NAT traversal. Reference implementation of Session Border Controller (OpenSBC) available.
* OpenSoftphone: A simple Java based SIP softphone using the PjSip-jni wrapper.
* OpenZoep: GPL telephone and IM messaging client engine
* Peers Minimalist SIP softphone written in java (tested on linux and windows)
* PJSUA: Command line SIP UA with SIMPLE, IM, call transfer, RTCP/RTCP, etc.
* QuteCom ex-OpenWengo: a fully SIP compliant multiplatform softphone with many features
* SFLphone, open-source multiplatform multi-protocol VoIP client * Shtoom: SIP softphone in Python, runs on Windows, Mac, Linux * SIP Communicator Audio/Video phone and messenger – Multiplatform – Open Source (also supports XMPP, MSN, AIM, Yahoo! and others). * SipToSis from mhspot.com Skype SIP UA – Multiplatform – Open Source
* sipXezPhone (“sipX easy phone”) from SIPfoundry based on sipXtapi * sipXphone from SIPfoundry, previously known as the Pingtel phone
* YateClient is multiprotocol and multiplatform softphone with H.323, SIP, Jingle and IAX support.
* YeaPhone: A SIP softphone for the Yealink USB-P1K handset based on the libLinphone backend