Leaving aside the positive v's negative stuff .....
Just to clarify, are you saying that the BT engineer (Ian) told you that there were different tones for engaged and off the hook?
No, same tone, but dependant on the technical state of a phone line when operator did her/his check up, the operator might be able to break into the line, or might have to wait until one party or another vacated use of the telephone they were in control of, at (a) or (b) - if the state of the phone line an operator was checking was Simply 'negative' (phone off the hook) the operator could break into the line (positive influence) because the negative situation was "matched' by influence of the operator,. Similarly, if the phone line had a 'negative' / 'positive' state, at the time an operator was making a check, the operator would be able to break into a line because the 'positive' influence of an operator checking such a line would be such that a Negative / positive / positive condition is met! A negative / positive condition was met, when a different set of circumstances were prevalent, such as in the following examples:-
Phone (a) is off the hook, and (b) the operator is trying to break into the line, which can be successful
Phone (a) and phone (b) are is use in the following set of circumstances, there has been a call in progress between phone (a) and phone (b), but that one or other of the caller's, then for whatever cause or reason they put the handset of one or other phones down, off its hook, and after this has been done, the operator decides to make a call to check either - either to called (a) or (b)..
But..
When phone (a) ends up with its handest 'off the hook', when either a call was in progress between caller (a) and caller (b), or vice versa, the state of the line between both parties would be a positive / positive condition, and the operator (positive influence) could break into the phone line, because the positive status of the line was greater to that required in order to allow an operator to be able to break into a telephone line, which is / are (1) a negative condition, (2) a negative / positive condition, (3) a positive / positive condition..
However, an operator would not be able to break into a telephone line, where none of the aforementioned conditions are met, where for example, phone (a) is simple off the hook (no connection to any caller) and phone (b) decides to call caller (a) . These circumstances create / produce a negative / negative situation which cannot be overridden, or broken into, because the total sum of influences would be negative / negative, and could not be at least matched by the positive influence of the operators actions!
Ian Manley (BT engineer)