Coder/expert for RTP protocol, Android/Java networking and perhaps VOIP

I have an Android VOIP application whose RTP connection code doesn't work reliably - from some reason it occasionally fails to transfer voice data between the caller and callee. So I need someone familiar with RTP protocol, Android/Java networking and perhaps VOIP in general to either fix the current code or create a new RTP layer from scratch. If you ever created a VOIP Android app that uses RTP protocol to transfer voice data you should be more than capable to fix this. Note that current code uses UDP hole punching technique to traverse NAT routers. And note that your job is just to take care of RTP related code, which is encapsulated in 4 classes. Everything else is in place and working.