Woize plans SIP based Symbian Woize client
By Dusan Belic on Thursday, April 6th, 2006 at 5:11 AM PST In Applications, Symbian
Woize will be available for Symbian OS phones! 
If you’re not familiar with Woize it is the company that develops Skype-like client allowing free phone calls from your computer, PDA or Windows Mobile smartphone. Symbian OS support is a great addition, as far as I can tell.
Some people argue that Woize is much better than Skype, but Skype still has the largest market share, making it the most useful VoIP client.
Next step – porting of Skype and Google Talk to the Symbian OS platform.
This not only one VoIP client, in fact there are alot of VoIP and VoIP SIP client for Symbian OS, the list of them in this post: http://symbiancorner.blogspot.com/2007/08/symbian-voip-sip-applications.html