title: Adding PTT option to android app
body: <p>I want to add to sipdroid a push to talk option
  I saw the Lumicall open source but I couldn't make it work!!
  I have no idea how to start...
  I have tried to start with rtp as a regular call but it's have a delay
  and I want it to work immediately
  I don't know if it's better to work with sipManager?</p>