[#2459] Make sure the stream is opened when the call is answered

Fix the issue concerning the audio which don't start when calling
the voicemail in IAX with the quick dial button

sflphone-common/src/iax/iaxvoiplink.cpp

@@ -831,6 +831,7 @@ IAXVoIPLink::iaxHandleCallEvent (iax_event* event, IAXCall* call)
                 Manager::instance().peerAnsweredCall (id);
 
                 // start audio here?
+		audiolayer->startStream();
                 audiolayer->flushMain();
             } else {
                 // deja connecté ?