While you wait for a Bose Mod to process your reply. It might be useful if you give your Bose A20 Headset Part Number and also mention which Aircraft Make and Model you are using it on. (There are different Microphone Types and there may be a mismatch which is why I suggest this.)
Mostly once installed, if it is the correct match for your Aircraft type, simply plugging in will activate the Mic if the Intercom system is on and Vox turned down. If no Intercom, a Test transmit on the Aircraft Radio should give you a noticeable 'Sidetone' of your Voice if both the Headset Control Box Volume Levels are up and also the Aircraft Radio Audio up.
Try another Headset if you haven't already in the same connectors and ensure it is not a Socket or Aircraft problem.