BUILD A VOIP APP.

I want to create a VOIP app. Where users can register, add credit to their account through a payment gateway and make calls to phone numbers worldwide. The amount added to their account would be deducted according to their use on a per minute basis( the admin should be able to set these call rates accordingly for different country codes). Incase you decide to do this with an paid VOIP API available online then I will provide you with the api for calling to phone numbers worldwide. You could use your choice VOIP API ( ONLY after i approve the api you suggest). The UI should be excellent and user friendly. It should be similar to VOIP apps available online. For more details send me a message. Delay in the project will not accepted. It should be completed within the time span decided.