Free Linux Softphones
For Linux users, here are free softphones (A soft phone is an IP telephone in software.) that will work for you and the link for free download. From Twinkle, designed specifically for Linux to iaxComm, a client designed for Windows with versions for Linux and Macs.
Free softphones for Linux...
Twinkle, a Linux based softphone for VoIP and instant messaging uses 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. Twinkle supports a long list of features and has the most professional feel of the open source softphones.
An open source VoIP and video conferencing application for GNOME. Ekiga uses both the H.323 and SIP protocols and supports many audio and video codecs. It is interoperable with other SIP compliant software and Microsoft NetMeeting. Ekiga was formerly known as GnomeMeeting.
Iaxclient is an open source, multiplatform softphone that has been written
to work with Asterisk, but does support other VOIP protocols (including
SIP and H.323). IAX's simple, lightweight nature gives it several advantages,
particularly in that it can operate easily through NAT and packet firewalls.
Although we show you the Linux softphone here, Iaxcomm is available for Windows