Can't transfer from OBi2182

[N7AS]

On my OBi2182 IP Phone I have the InboundCallRoute fork the call to my cordless phone which has a Localphone number on my OBi200. So I can pick up the call on either the 2182 or the cordless phone. Whichever I pickup first takes the call.
At times while on a call on the 2182, I would like to transfer the call to the Localphone number on my OBi200 but when I hit the Transfer Button, I get this message on the 2182 screen "Call Error: No Call Key available to make another call".

How can I make this work?