Truphone Free voip calls on your mobile phone .

Truphone is another new free Voice over IP application that allows you to make free mobile calls to other Truphone users and very cheap calls to anyone else through WiFi , 3G or GPRS networks and the sound Quality is really great . like Skype

Truphone supports Nokia's E series, N series and it will soon supports more Nokia models including N90, N92 and N93 phones.
This application come already with the nokia N80 internet edition .
I had to update my Nokia e61 to make it work , and its really working more than fine .
Anyway I think you should really try it cause its more than great .
Truphone is available for free, either ‘over the air’ or via download from Truphone