I use an RCA "commercial" 2-Line Dect 6 and it "sounds like" half-duplex. There aren't that many 2-Line cordless phones out there (especially Dect 6) to choose from.

Also, this is the only one I have found that has a bright blue message waiting indicator that sits on top of a tiny lighthouse on top of the phone. Most other phones I have seen have easy to miss MWI indicators that require you to actually look at the LCD screen on the handset to check for messages. This MWI lights up the room and you can't miss a waiting message.