The telephone handset left off the cradle is in my opinion very revealing. There was no reason why it should have been anywhere other than on its cradle. When Jeremy rang Goldhanger from the farmhouse he had to leave the handset off the cradle so that his own phone in Goldhanger would continue to ring long enough for him to be seen to answer the call.
Having done so and then hung up he had unintentionally blocked all incoming and outgoing calls to his own phone. Thus a reason for the mysterious delay in phoning the police. Truth is, he had blocked his own line until the automated system in the old exchange cleared it.
It is possible that Jeremy phoned himself and then answered but he could have used his answering machine to accomplish the same thing then went home and erased the tape. It is also possible he didn't call himself at all and simply claimed that the phone was used.
The other main reason for leaving the phone off the hook would be to support the bogus narrative he told.
He claimed the call was cut off. He wanted police to believe that Sheila found him on the phone, disconnected the call and attacked him at that point. So he thought leaving the phone off the hook supported her attacking him as opposed to him calmly hanging the phone up.
Jeremy thought police would believe Sheila grabbed the gun from the kitchen table (that is initially where he told them he left it) she threatened him, he called Jeremy then she attacked and killed him there and went upstairs killing everyone else. As it turns out though we know that the shooting started upstairs. Neville was shot upstairs then went to the kitchen while wounded.
So the whole notion of him using the kitchen phone to call Jeremy before any shooting started is not credible at all. He didn't make it to the kitchen till he was already wounded. It is just one of the things he didn't realize police would figure out.
