I disagree, because the operator could not patch through an exchange line call from whf in that era, because of the old type of exchange system, the only way the operator could patch the line through from the farmhouse to the police using the 999 system, was if Nevill's call (or at least one of them) had in fact been a 999 call! I was told this by telephone engineers
There was obviously some misunderstanding. If the WHF line had completed a 999 call (by hanging up) and then been left off-hook, the prior 999 call would be irrelevant to the current status. If the 999 call hadn't been completed, the operator wouldn't have told Pc West that the receiver was simply off-hook. The operator made a statement in which she said that she connected the WHF line to the police HQ without using the 999 system, because she wasn't allowed to use the 999 system for that purpose.
Pc West may have used a second line to try to make that call, hence why he would get an engaged tone!
It doesn't matter what line Pc West was using, as the engaged tone implies that the WHF line wasn't available. The operator told Pc West that the engaged tone was because the receiver at WHF was off-hook.
Failing that, Nevilles 3.26am call may not have changed into a 999 status until much later in the early hours of the morning
There is no evidence that the WHF line ever had a 999 status.
. . . the operator reported she could hear a dog barking when she reported the phone at the farm being off the hook
At 3:42am (according to Pc West's log), the operator told Pc West that the explanation of the engaged tone was that the receiver at WHF was off-hook. She didn't mention hearing a dog barking until later on, after another line check had been requested. On that later occasion, the line was still engaged due to being off-hook, but there was a new piece of information, that a dog had been heard barking. The line's status hadn't changed. What the operator could hear, whether a dog barking, sounds possibly caused by the raid team, or anything else of that nature, had no bearing on the line status. The line remained engaged because it was off-hook. Whether the police happened to write "off-hook" or "engaged" in their log on each occasion is irrelevant.
. . . why would it suddenly become engaged at 5.47am?
It didn't suddenly become engaged. It was necessarily engaged at all times when the receiver was off-hook.
. . . at 6.08am...
The police log suggests that the police asked the operator to switch the line to the police HQ using the 999 system, but the operator explained in her statement that she connected the WHF line to the police HQ using a normal exchange line, not the 999 system (which she wasn't allowed to use for that purpose).
. . . somebody inside the farmhouse made a 999 emergency call requesting ambulances
I've seen no evidence that the WHF line was used to make a 999 call for that purpose or that any 999 call was used to request the ambulances. However, the police could have asked for ambulances to attend using that line whilst it was off-hook and being listened to by the operator. I haven't seen anything to indicate that occurred, but it's technically possible. I think it's more likely that the police at WHF used their radio instead.