Seriously! One is using Verizon on a Blackberry Curve. One is using T-Mobile. Another AT&T on a Nokia E61i like me. The first is not interested yet in using mobile VoIP. I need to check out her Blackberry to see if it is capable even. The second two are using Truphone, Fring, and checking out Nimbuzz since my blog comparing mobile VoIP. One of them uses Celtrek because she travels all over Europe and Latin America and wants the reduced roaming fees and local phone numbers, something that makes Celtrek very unique.
The only reason the other 2 still have a landline is that Cox has convinced them that they have to buy cable, Internet and phone all in one from them where the phone service is app. $35 USD/month and does not offer unlimited calling to anywhere in the USA. I'm not one to force anyone to do anything. Lead by your example.
So read this blog post at Joop, very professionally done ... http://www.joopdorresteijn.com/Archive/why-embracing-mobile-voip-is-vital-for-carriers/
... browse some of our team's other blogs at http://blogs.didx.net and http://blog.tmcnet.com/monetizing-ip-communications/. You're welcome to comment, contribute, and collaborate any time.