[Freeswitch-users] RTP traffic on LAn

Hi all, I'm trying to find a way to let RTP traffic between two phones, registered to a public Freeswitch, on the same LAN, remain local. Usually phones are natted behind an ADSL router and using two RTP streams to speack each other (because, for example, are on different  rooms on the same office) consume a lot of bandwidth. Is there anyone who has found a solution to this problem? I was thinking on using SIP INFO for sending DTMF (in case customer have to use some phone features activated via phone), using private IP (no STUN on phones) and keeping FS out of RTP streaming. Any help will be appreciated. Chris