Software base VoiP call termination from Mobile phone and connected to server with interface

We are looking for solution like a traditional GSM or CDMA VoiP gateway. This project will be separated in two parts. One is mobile application and another one is registration server. The Mobile application will register to a server and accept call from that server with IXA or SIP protocol. After that call will terminate to a GSM network. (this part just like a traditional GMS getaway) This mobile application will work on wifi Internet connectivity and mobile Data internet , All call will pass with G729 codec The registration server may be Asterisk or VOIP switch or any other server or customize server. This server will receive call from another VOIP switch server with SIP protocol. Certain number of registered Mobile will be able to assign in a group of gateway. On server have to include option to show balance through USSD all cell phones will act as a port like any normal voip gateway . the application should be connected to interface to manage all ports . security should be high on the server . the project should accept any number of new cellphones added to interface . also any new cell phone should be given a security key to allow it to be added in my server .