[Freeswitch-users] Port question again

Hey Guys, Thought the port question was asked a number of times, I couldn't find an answer to this. So please bear with me. I have a Freeswitch box that is on the Internet without any sort of NAT. I want to block as much ports as possible on this box while still allowing Freeswitch to 1) receive calls from Voip providers and 2) send calls to other VoIP providers. What port can I block and what ports do I need to let open? I know 5080 needs to be open. But can I restrict the RTP ports to, say, only 20000? Thanks so much. Max.