How to make Android VOIP App using REST based API of SIP Server

I am totally new in VOIP area.

My client has a API which is based on REST. My task is to create a Android app which enable the user to call using WiFi or Cellular Network and the SIP Server of my client. I also have to fetch user's data and perform some other functionality using that API.

Please suggest me the working procedure that I should follow.

I have viewed SIP Demo provided by Android Developer's Website but I guess it is not the thing i am looking for.

I've also asked about which codec to choose: Which is the best SIP compatible codec type for Android

Solution

The API should be used to get data from the SIP server. API itself cannot send the packet stream and data between the caller and called. RTP Stream is used for handling the voice data. So, the working procedure should be:

Initiate a SIP Session using the SIP Server
Create RTP Packet and send through the server to the called person
After terminating the call, use the REST API to get the information of the call etc