Well, I've done it the way you want to do it many times in the past by disabling in COS. The flyover says: "Permission for others to change this phone from ringing to handsfrr answer back".

BUT....

I just tried it on our inhouse system runing 5.25, and another 3.10MK063 system, and they both work the way you said. The COC disables the ability of that station to change other phones to handsfree.

So I don't know what it going on with it because I have done exactly what you want to do several times in the past.