Nokia was offering VoIP client application in a few N series models but in my experience VOIP application doesn’t work in N series phone. As I had a Nokia N73 and that has SIP client, I tried alot to configuring it but that reallly didn’t work for me, later on I came to know that that is just dummy program and will not work 
and I also read somewhere i.e. Nokia models N78 and N96, Nokia removes predinstallirovannoe application for voice over IP-client, until recently, the attendant at all N-series phones. If you have E-serious phone, VoIP client will still work for you..
A for as VoIP on N-series conceren The client application allows users to make calls over the Internet, not only with traditional cellular communications. Nokia confirmed that the SIP protocol for its mobile phones will continue to support the projects VoIP, but in order to carry Internet calls to owners of phones N-series based on S60 3rd Edition Feature Pack 2 will have to use other VoIP-customers.
“Native” VoIP-client Nokia will remain a business E-series devices.
