I remember vividly discussing this aspect of the investigation with Jeremy when I was incarcerated with him on B Wing at HMP Full Sutton on several occasions during late 1989 and July 1990...
Jeremy said that he answered his phone after two or three rings when his father called him that morning!
He said therefore the automatic recording mechanism on his answer phone did not get activated! As such there would have been no recording what his father said to him! He said he told the police that he had answered the phone call quickly..
However, he also suspected that the police thought that if he had been the killer, that he might have telephoned his own house number, so that his answer machine had recorded the connection from the farmhouse, or a snippet of conversation that had been fraudulently disguised to sound like it had been his father who had made the call to him, when in fact he might have made the call to himself! We talked about the workability of such a theorey! I said if he had been the killer, at the house making a disguised call to himself, it would have been rubbished Almost immediately because Jeremy was supposed to have answered the call from his father, not recorded it on his answer machine! Jeremy said he thought the police were hoping to find a recording to usurp his account but that they didn't find anything or they would have used it against him! I got the clear impression that Jeremy hadn't tried to use his answer phone to prove the call he received from his father had in fact been made! In any event, the phones at white house farm and Jeremy's cottage at Head Street, Goldhanger were bugged and every phone call made and received at both locations were constantly being monitored under a telecommunications Act warrant - the drugs squad who had Jeremy and his cronies under surveillance, knew that a call had been made from white house farm to Jeremy's cottage, before Neville Bamber had made his call to Chelmsford police as per the phone log timed at 3.26am! I think the exercise of the cops seizing Jeremy's answer phone and five audio tapes was simply done for taking purposes, since Neville Bamber almost certainly phoned the police at 3.26am, some 10 minutes before Jeremy himself ended up phoning them at 3.36am...
I spent many a long hour puzzling over the phone call issue..
I took advice from BT engineers, one who was retired, another who was currently working for them...