I've been using Truphone on my N80ie for the past couple of days and I am impressed. Now I know I said I was impressed with fring (and I am) but Truphone has better call clarity.

So what is Truphone?
"Truphone routes your mobile calls via the internet. That means free mobile calls to other Truphone users & very cheap calls to anyone else."
Currently when you sign up you get a £2 credit and free landline calls to 40 countries, including the UK

I'm using it over my home wifi connection. I'm scared to try it over 3g as I'm still on O2.

It's currently only available on the newer N series and E series phones.

It's also free to call other SIP users. I think it's definitely worth a try.