Develop a SIP Phone App for iPhone and Android to integrate with A2billing

I need someone to develop an App for both iPhone and Android to integrate with A2billing and Asterisk. The functionalities that I expect of the app are as follows: - Show Credit Balance - Ability to make Calls (Off Course) - Show Call History - Show Payment History - Change Password - Ability to Recharge using either a credit card or Paypal - SMS (looking to add an SMS gateway) - Chat (this is needed only be to chat with the support/helpdesk team) Other functionality will be for a local address book within the app. The ability for users to set up favorites (people, family and friends that they call often). Our company logo will be displayed on the app. I am looking for something that is quite easy to use, and also visually clean looking. In other words, not to complicated. Drag and drop features will be welcome. For example, the ability to drag a number from the "Call History" into the favorites icon thereby adding it to your favorites. I will also need an easy documentation on how the app works. Anything else that you would recommend to make it better would also be appreciated.