The two possibilities pre massacre regarding what Bamber knew on the technology BT/the police had to track phone calls are -
He was (correctly) 100% certain BT/the police could not obtain information on whether a call was made/answered from WHF/Coldhanger.
He did not know what information BT/the police could obtain.
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If it was the second situation, his AM gave him the confidence to commit the massacre.
He could -
Phone from WHF.
Have his call answered by his AM.
Terminate the call at WHF.
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Finally the penny has dropped!
So what you're saying is that Bamber could have created an audit trail of a tel call from WHF to GH by phoning an AM at GH from WHF.
1. On the assumption that he genuinely believed BT/police could trace a tel call retrospectively how could he be sure that having a call go into an AM would leave an audit trail in the same way as lifting and replacing the receiver did?
2. If he did the above it would mean he would need to account for the time delay between making the call at WHF and then phoning the police.
3. There's no evidence he had an AM pre murders but if he did it would have been in his interests to destroy it because not having it switched on would seem somewhat convenient and I am sure the prosecution would have argued he made the call to an AM to create the audit trail at the exhange evidenced by the timestamp of the call in the exchange and his call to police ie time spent getting from WHF to GH.