Android Push-to-talk application

Our company is looking for an expert Android programmer with high skills in Voice over IP. The application that we want to create must provide Voice over IP communications in two modes: like a regular telephone call and in Push to Talk mode (PTT). Applications requirements are: • Real-time transmission mode • Voice over IP and PTT must complain the SIP standard • The service will include a server side for authentication. All the voice packages between terminals must pass through the server. Server side solution is not the purpose of this work, but suggestions are accepted. • The application must be able to use mobile data (whatever is available: GPRS / EDGE / 3G / HSDPA / LTE) to be usable everywhere, but also must be able to use wi-fi, to reduce costs and mobile data consumption when possible. • Voice packages can be encrypted. • Reliable delivery. That means, without voice loss and stuttering, even with difficult and intermittent network coverage. This implies automatic re-transmission until a voice message is successfully transmitted. • Text messaging integration. • Optionally using the internal speaker for playback to protect the sender's privacy when listening to messages in crowded spaces. • Screen design must be included. • Source code shall be delivered. The server side of the project is being developed using Open Source IMS. Your bid must include: • How long it will take. • References for similar works.