Ekiga!

Riot (formerly Vector) is an opensource interoperable app running on Web, iOS and Android. It is built around chat rooms, both public and private and provides messaging, filesharing, voice and video conferencing (beta on Android and web), integrations to tools like GitHub, Jira, Jenkins and much more soon. Vector is built on the Matrix.org open standard which provides bridges to other systems such as Slack, IRC and a lot more soon. One can use the hosted service or run their own. »

Linphone is an internet phone or Voice Over IP phone (VoIP). With Linphone you can communicate freely with people over the internet, with voice, video, and text instant messaging. Linphone makes use of the SIP protocol, an open standard for internet telephony. You can use Linphone with any SIP VoIP operator.

With ooVoo you can make free, high-quality video calls, voice calls, and send text messages to easily connect with your friends and family. Make clear and crisp video calls with up to 12 people at a time, on any device. Try it now!

Nimbuzz offers limited availablility free calls and limited availability free text messaging for users via 3G and WiFi. Calls to other Nimbuzz users are free. PAY to call mobile or landline phones for as little as $0.02/minute in bulk with NimbuzzOut. Free SIP to SIP calls.

Technitium Bit Chat is a secure, peer-to-peer (p2p), open source instant messenger designed to provide end-to-end encryption. Primary aim of developing this instant messenger is to provide privacy which is achieved using strong cryptography. It can be used over Internet and private LAN networks for instant messaging and file transfer.

csipsimple will allow native sip for android device.

Zoiper IAX & SIP multilanguage and multiplatform (Windows, Linux and Mac OS X) softphone is a VoIP soft client, meant to work with any IP-based communications systems and infrastructure.

What does it do: