These all are SIP clients which means they all can be used to call each other and they just need a SIP address (like your mobile number) . You can also register to a commercial VoIP provider(which supports SIP) to place calls to normal phone line or cell phone .
You can get a free SIP address from here.
Ekiga (available in the software center)
Ekiga (formely known as GnomeMeeting) is an open source SoftPhone, Video Conferencing and Instant Messenger application over the Internet.
PC to Phone calls in ekiga .
The default provider is Diamondcard
Worldwide Communication Service, which
offers these rates
To purchase the account, go in Tools -> PC-To-Phone Account. You
absolutely need to purchase the
account from the PC-To-Phone
configuration window or it won't work.
Once you have opened an account, login to your account to look up your
Account Number and PIN. Don't confuse
these with your username and password!
Enter the account number and PIN in the appropriate fields in the
window and enable the PC-To-Phone
If you need to edit the account under Edit->Accounts, you should use
account number and PIN here also, even
though it says "user" and "password".
You are now ready to do PC-To-Phone
calls at very low rates all over the
To dial a number, simply add "00"
followed by the country code, and by
the phone number you want to reach.
For example, sip:003210111111 to call
number 10111111 in Belgium.
SFLphone (available in the software center)
SFLphone is a robust standards-compliant enterprise softphone, for desktop and embedded systems. It is designed to handle several hundreds of calls a day. SFLphone is available under the GNU GPL license, version 3.
Twinkle (available in the software center)
Twinkle is a softphone for your voice over IP and instant messaging communcations using the SIP protocol. You can use it for direct IP phone to IP phone communication or in a network using a SIP proxy to route your calls and messages.
Yate VoIP (available in the software center)
YATE is a telephony engine aimed at creating a telephony server that performs well enough to deal with PBX requirements and also flexible enough for complex Gateway and IVR solutions.
YateClient uses the same telephony code to produce a software desktop VoIP (SIP, H.323, IAX2) phone. yate-qt4 uses this to provide such a telephony
QuteCom (available in the software center)
QuteCom is a community of enthusiasts and developers, creating free software products related to communication over IP. The flagship product of the QuteCom project is a softphone which allows you to make free PC to PC video and voice calls, and to integrate all your IM contacts in one place.
Linphone (available in the software center)
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
Linphone makes use of the SIP protocol , an open standard for
internet telephony. You can use
Linphone with any SIP VoIP operator,
including our free SIP audio/video
Jitsi - Open Source Video Calls and Chat
Secure video calls, conferencing, chat, desktop sharing, file transfer, support for your favorite OS, and IM network. All this, and more, in Jitsi - the most complete and advanced open source communicator.
A state of the art, easy to use SIP client
Available for Mac, Windows and Linux