Hand held com. issues

What am I doing wrong? I am trying to use an ICOM A-20 and or an A-21 Hand held through a Sigtronics Intercom. I have tried both an A-20 and A-21 with a Sigtronics SP012 and a SPO22. I am using a ICOM headset adapter. What happens is, when I connect the Interphone to the headset adapter all goes well until i plug in a microphone then the radio goes to transmit. This is with or without the PTT being plugged in and with the interphone power switch on or off. Those dad ratted electrons are sneaking to ground somewhere. This must have happened to someone smarter the me, who figured it out. Help!