I also had it explained by a BT engineer and from BT archives. They wouldn't have needed an operator to break into the call if a tone distinguished off the hook from engaged.
You were misinformed, or you misunderstood what you were being told - since, back in the day, an operator could break into a line with its handset off the hook, or even when a live call was in progress between two parties, but in the circumstances involving one party having their handset off the hook, and the other party trying to ring their number, the basic equipment at the local (exchange) substation became 'overloaded' and prevented the operator from breaking into the off the hook line, or the caller's line, because the situation created an impasse in the electronic circuitry at the substation. The operator would have to wait until either one of two conditions was met before he / she could break into the line still in its active condition, i.e. the phone off the hook, or the caller still trying to contact the phone which was off the hook! I had the benefit of access to two bt engineers, one who is a personal friend of Jeremy's (Ian), and a retired BT engineer who was a specialist in the workings, analysis of the metering system in use at the substation (exchange) of concern...
I was satisfied with the information I received at the time, and knew that behind the scenes, Essex police could have proved, or disprove the claim made by Jeremy that he had received a call from his father, when Jeremy had said he had, and did...
Essex police had the means to put Jeremy's script to the sword, but they were unable to do so, despite having the technological know how to go about it!
On top of this, Essex police had the drugs operation surveillance evidence gathered during the period when the shooting tragedy at white house farm unfolded - the drugs squad had low flying aircraft taking video footage in the vicinity of white house farm, and Jeremy Bambers Head Street, Goldhanger, cottage, on 6th August 1985, onward. They had telephone tapping evidence obtained under a warrant (telephone communications act), and were actually monitoring the phones at white house farm, Head Street, and Julie Mugford's residence, when the shootings were taking place, so there was actually no need for Essex police to have to unravel the metering at all the locations mentioned, because the drugs squad had all the evidence confirming this at their disposal, already!