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Here is a list of some of SIP clients for Nokia S60. Kindly note that these are generic SIP clients and not bound to any SIP provider as such.

• Tivi - Tivi is perhaps my favourite softphone. It is available as a free download and I believe the authors intend to keep it this way. Moreover Tivi is perhaps the only softphone which can directly call an IP address without registering with a SIP provider (though I couldn't hear the opposite person when I tried this feature).
• Fring - The main disadvantage of Fring is that it routes all SIP calls through its own proxy server. So there is latency for calls made using SIP.
• Adore Softphone - To use Adore Softphone, you will first have to register your device's IMEI number. Then you can proceed to download it. Ascent Telecom's Endura Mobile Dialer and REVE Systems' Itel Mobile Dialer Express are similar to this but they are not available for free for end-users.
• Talkonaut - I did not try this but I think this works based on callback.

There are other mVoIP clients such as Truphone, O-FONE, etc. which all use the SIP protocol but they do not allow configuring other SIP providers and are bound to their services. There is a possibility though that these products may in the future enable to configure other SIP providers.

Lastly if you are likely to use mobile VoIP extensively, you may consider buying a Nokia handset which comes with a built-in VoIP client. (See References below).

References:

http://en.wikipedia.org/wiki/Mobile_VoIP#Software_clients
http://wiki.forum.nokia.com/index.php/VoIP_support_in_Nokia_devices#Supp...
http://symbiancorner.blogspot.com/2007/08/symbian-voip-sip-applications....