voip:
Tutorials (by @symnok):
• Symbian 9.2 / 9.3 (Livejournal)
• Symbian 9.4 / S^3
VideoGuides
• Youtube (Peter)
More info
NokiaVoip (InstantView)
Apps:
• S^3 / Anna / Belle
• S60v5
• S60v3 FP2
• S60v3 FP1
Alternatively, you can also try the 3rd party crossplatform Mobile VoIP client, which supports a range of brands that use its own operator.
• Modded installer of latest version by @iCrazyDoctor
(avoids error "built-in component" if Nokia SIP is also pre-installed)
(supports only list of its own pre-defined brands)
• Web
• Old versions