IMO, setting DMZ or changing any other RG settings will not fix either of your two specific problems, though they they may help with other issues.
The beeps heard by the remote party are known as "DTMF talkoff". Southsound has a fairly good explanation in
http://www.ooma.com/forums/viewtopic.php?f=4&t=8068 . However, the OBi is a lot more flexible than Ooma and there are some settings that may fix it for you. For ITSP Profile A (or B, as appropriate) -> General, try changing DTMFMethod from Auto to InBand. This should eliminate the beeps, but may make DTMF control of voicemail or IVR systems less reliable or even non-functional, so test that with some systems that you regularly use. If no luck, undo that change and try setting Phone Port -> DTMFRxMode to software. That may not eliminate the beeps, but they should occur less often. Again, test to be sure that IVR access isn't broken.
I have U-Verse at home (i3812V gateway) and have no VoIP troubles. I doubt that your problems, at least the ones mentioned so far, are related to AT&T. To confirm, take your OBi to a friend's or neighbor's with a different ISP. Or, try X-Lite there.
Your other issue, poor double-talk performance, plagues VoIP in general. It's unfortunately quite complex. The Introduction to VoIP document referenced in
http://forums.whirlpool.net.au/archive/1516052 is reasonably non-technical. If you are familiar with the terminology, see
http://www.cisco.com/en/US/docs/ios/solutions_docs/voip_solutions/EA_ISD.html for a more detailed explanation.
AFAIK, there is no way to greatly improve OBi's handling of double-talk, but if your outbound voice is heard louder than needed by your contacts, lowering Phone Port -> ChannelRxGain will reduce the effect of your outbound voice on incoming audio. If you have a good headset, try turning off AEC in X-Lite. If the result is acceptable, getting a decent IP phone should give you similar results with a regular handset. As a bonus, IP phones don't have DTMF talkoff problems.