Android : How do i create primitive voip app with encryption and decryption operations in android

I am an embedded system engineer and has no prior exprience in Android. I need to design a simple VoIP app for my graduate study project. I have searched on internet found some docs about SIP and RTP but really do not know how do i implement these protocols. Here is the outline of my project. User log in the app by id and password. App accesses the contact list of android device. Each contact has a single master key which creates all required authentication and encryption keys.Let say, "John" is caller and "Martin" is callee and both use "Banana" as a master key. SRTP uses "Banana" as master and creates all required keys. When "John" touches "Call" button for "Martin", calling starts. When the call reaches "Martin" 's devices a box with textview pops up and requires master key for incoming call. If "Martin" enters "Banana" as master, authentication key are generated and RTP payloads are decrypted. This is whole scenario that i want to implement. I would prefer to use already written SIP, SRTP and key generation algorithms code by someone elses.

Thanks in advance.