Zoiper

Zoiper (an evolution of Idefisk) is a small and easy to use softphone application compatible with IAX and SIP protocols.

It allows configuring multiple SIP/IAX accounts and has multiple lines (up to six simultaneous calls).

It has all the features expected in a softphone, like for instance an address book, hold, transfer, conference, and voicemail buttons. Additionally, it has a send fax option (it supports T.38 fax over IP), and compatibility with lots of PBX and Voip Service providers through its SIP and IAX profiles.

This softphone has a very simple interface but it allows for advanced configurations. It supports the following codecs: u-law, a-law, GSM, iLBC 20 and 30, speex, and G729 (with an additional cost). There are versions available for Windows, Linux, and Mac.

ZoIPer supports the following languages: English, German, French, Spanish, Dutch, Portuguese, Russian and Japanese

I definitely recommend this software for advanced users and for enterprises.

It can be downloaded as a demo (less features) for free and upgraded through a commercial license.