app that can make phone calls over the internet (Voip)

I want to create an app that can be downloaded to a mobile device (or desktop) that can be used to make phone calls over the internet (Voip). This is similar to “Whistle Phone” and “Text Free with Voice”,), Additional functions would include text messages and emails all from a single platform. People would either go on to our website, the I tunes App Store, or the Android Marketplace to download the app. Once this is complete they will have to register,buy credits.I have a VoIP gsm gateway that will have sim cards to connect the user to the mobile phone. For layout/construction ideas we would refer you to the following apps and websites “Whistle Phone”, “Text Free with Voice”, “Magic Jack” (both the website and app)”, Google also has a version in their Gmail under “Chat / Call Phone”, & possibly looking at “Vonage” would provide additional direction. Eventually the app will have to be functional on desktops as well as phones