59PA, you are asking the key question, that for some reason, the answer to which eludes many pilots, operators, and POI's.
I've never understood why people get twisted up into the discussion of an "on call" term not being in the FAR's. It doesn't need to be and IMHO this was by design. Instead of trying to define any and all possible terms an operator might come up with to describe being "on call", the FAR's were written specifically to define rest requirements and what qualifies as rest. Subpart F specifically and completely lists all applicable Rest Requirements per operation type and 135.273 specifically and completely defines Rest / Rest Period.
Secondly, The FAA has issued numerous letters, notices and publications addressing this matter. But for whatever reason, many pilots and operators still remain confused and continue to argue about the "on call" term and likewise many POI's can't seem to read and understand the language either! That is why the "rewrite" is being done, not to close a loop-hole, but to clarify there isn't one to begin with and to stop operators AND pilots from trying to exploit their "misunderstanding" of the FAR's.
With all of that said, understand that being "on call" 24/7 is not in and of it self what is illegal and unsafe. There is nothing wrong (from a regulatory sense) with being "on call" for any length of time in order to be given a schedule or assignment which will provide for a PROSPECTIVE Rest Period (as defined by the FAR's) within any preceding 24 hours. The bottom line is that any time that you are "on call" is not and can not be considered Rest or any part of a Rest Period.
BTW, keep in mind that this is only addressing the regulatory issue and we aren't even discussing the practical and physiological reason for why these rules are present and need to be respected by operators AND pilots. The whole point is to avoid flying fatigued. Never forget that it is our responsibilty as pilots to operate safely and the lack of understanding or concern for current regulations by your operator or lack of government oversight shouldn't relieve you of common sense and safety.