Looking for help with an odd problem. I just got a new Rexon RHP-530 Com handheld radio. It comes with a headset adapter cable for use with aviation headsets having the standard 2 plug connection. The radio has side tone which the manual says is automatically activated when the headset adapter plugs are inserted into the radio sockets. Surprisingly with I tested the radio with my Lightspeed Zulu 3 headset it had no side tone. It broadcast and received fine, but I could not hear my voice during broadcasts. So I tested it with 3 other Lightspeed headsets and got the same results (a Sierra, a QFRXCc active noise reduction headset, and a QFRXC passive noise reduction headset). Next I tried an ASA HS1A PNR headset and it worked normally (got the side tone)!. After that an old Sigtronics headset and it also produced side tone??? I called Gulf Coast Avionics, who sold me the radio, and they tested the same model radio with headsets from Lightspeed, David Clark, and Bose. No side tone with the Lightspeed and normal side tone with the DC and Bose. The radio works great with any headset not made by Lightspeed. Neither Gulf Coast nor Lightspeed could explain how this could be. So I am reaching out for help to POA. Anyone seen this before? What are possible causes and fixes? BTW, the radio has a side tone volume adjustment. I tried the full range of settings with my Z3 and it had no effect. I also tested with the headset switches in both Stereo and Mono. No change other that in Stereo there is only sound in one ear cup.