SIP to Whatsapp Gateway

We need to develop a SIP to WAPP gateway. The gateway should be able to pass voice calls incoming over SIP and forward them through WAPP to complete the call to the called party number. The development platform/operating system is not important. The project should be completed either by using the Linux/Windows WAPP executables, or by using the Android / Windows Phone mobile versions of the application, no matters on the version number. The implementation should return the correct call error codes to the SIP backend, i.e. CALL SUCCESS, BUSY, UNAVAILABLE, etc. For a successful project, we’ll select the one, triggering successfully continuous SIP/Viber calls.
Functional flow
1) Calls originating will send to WAPP gateway
2)WAPP gateway converts the sip/iax signal to WAPP protocol
3) the termination number carried from the origination header will be checked by the Asterisk gateway , if the number is used by WAPP and if the number is online, the call will terminated on WAPP!.
4) if the number is not used in WAPP it sends 503 error and rerouted to other gateways .