title: No ringback for outgoing calls sip
body: <p>I'm developing a SIP client app using Android 2.3+ API, I can make a outgoing call and receive a incoming call successfully by using SipDemo sample codes. but I can't hear any thing before peer answers the call. </p>
  
  <p>What steps will reproduce the problem?</p>
  
  <ol>
  <li>register to sip provider (mSipManager.open())</li>
  <li>place a call (mSipManager.makeAudioCall)</li>
  <li>silence (onRingBack method trigger in SipAudioCallListener)</li>
  <li>other side answer (onCallEstablished method trigger in SipAudioCallListener)</li>
  <li>both sides hear each other with good quality</li>
  </ol>
  
  <p>I expect for ring-back tone, in step: 3 instead silence.
  Can i do something in onRingBack for hear sip ring-back tone instead of silence.</p>