Skype is a VoIP software that also provides QQ's instant messaging function. Microsoft just bought Skype. The originally unfriendly Linux Skype does not know whether it will completely insulation Linux users, so I searched for alternative Skype software on the Internet. Note that all VoIP Software under Linux is free of charge!
1. Ekiga
The most famous VoIP client in Linux, the former GnomeMeeting, supports the SIP and H.323 Video Conferencing protocols and is fully open-source and can interoperate with Microsoft's NetMeeting. However, unfortunately, it does not support Windows Meeting Space, a substitute for NetMeeting.
Download Windows Version: http://down.51cto.com/data/206819
Download Linux source code: http://ftp.gnome.org/pub/gnome/sources/ekiga/3.2/
2. GNU Telephony
It is now in the 1.0 stage. With the release of the gnu sip Witch 1.0 SIP server, it is another step away from end users. Now it is only suitable for software developers to study, however, this open-source project is worth looking forward.
Download source code: http://www.gnutelephony.org/index.php/Secure_Call
3. Google Chat/Google Talk/Google Voice
If you are in the US, you can use it.
4. Jitsi
Previously called SIP Communicator, which may be the most functional VoIP client in Linux. It supports the SIP and XMPP protocols. It is available on Windows, Mac, and Linux platforms and is currently being transplanted to the Android platform, it is written in Java.
Download link: http://jitsi.org/index.php/Main/Download
5. Linphone
Linspire Linux only supports the SIP protocol and Windows, Linux, and Mac. The next version will support Android, iPhone, and BlackBerry.
Download link: http://www.linphone.org/eng/download/