Android VoIP and PTT 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: • Must work with Android versions 2.3, 3 and 4, this include phones and tablets with screen sizes between 7” and 10”. • 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. • Conversations must be possible between GPRS, UMTS, HSDPA, WIFI, so adaptive codecs must be used. • Voice packages must me encrypted. • 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.