I’ve been playing with a softphone called Ekiga for a little while and I can recommend it. It’s open source (free!) and is available for both Windows and Linux. I tried the Windows version.
Installation is quick and simple and entering the extension details for my Asterisk server got it registered without any problems.
One of the most interesting things is the variety of codecs that Ekiga supports. The ‘default’ for most people when using Asterisk is G711, an 8kHz codec. This should give the same quiality as a ‘regular’ PSTN telephone call.
Ekiga supports G722, a 16kHz codec, and enabling this, along with a pair of USB headphones (Plantronics Audio 655) it was definitely possible to hear the difference. Audio on calls sounded clearer and fuller. This obviously assumes that the call is in G722 end-to-end. If it goes on to the PSTN audio will be converted to a lower quality there.
Maybe even more interesting is the support for CELT. This is a very high qulity stereo codec. While this is not currently supported in Asterisk, version 10 will offer pass-trhu support for CELT (Asterisk 10 Audio Codecs) and, in the future, hopefully be able to transcode it too.
So if you’re looking for a simple, quick, cross-platform SoftPhone then it’s worth checking out Ekiga – ekiga.org