If you transfer a call, it puts the initial caller on "hold", then (as part of teh transfer)the phone grabs an intercom button....if you dont have an open interom button, you can not transfer...
if you only have 1 intercom button, you can not possibly transfer the call....
Your first statement though, is misleading...only 2 call appearance keys has one call on hold or park and an active call,.....
This changes things. If you only have 2 intercoms, and you already have a call on hold, you will not be able to transfer....BUT, if you have a call on hold, and another call comes in (2 intercom buttons) then you would need to PARK 1 of the calls, then go back to the other caller, and now you would be able to transfer as you haev an open appearance button to complete the transfer.