Well, there is rather tricky way to do it, but you can not have any Calling Groups other than Voice Mail, and the OPERATOR has to make it happen. (Or the user, if they are on an Operator Position).
In fact, a 10 button set could work.
You would have to use Extension Status and set the system up for HOTEL Mode Ext Status.
Then an ES2 button could be placed on their phone (Like on one of the HIDDEN SPEED DIAL BUTTONS #01 to #24 - depending on the set).
This would let the user toggle his status from AVAILABLE (Not able to dial) to OCCUPIED (Able to dial).
Again, this can only be done on a phone in an OPERATOR port, and the System ESS Mode has to be HOTEL.