I'm shooting from the hip here, but if I'm not mistaken the 2-prong adapter/plug has a handset RJ (what is that, RJ22?) in the end of it that you can remove the handset and plug in the wireless headset. However, you cannot use the handset lifter beacuse you have to press a button to answer.
If your 2-prong adapter doesn't have the RJ then check with someone like
Walker Handsets by Clarity as I KNOW that those adapters exist.
regs,
.al.