It's not possible to know exactly how things played out. If could see the full contents of the original SC file, it might help a bit.
I agree we cannot know exactly what happened, but we can look at the evidence and make logical inferences.
My scenario addresses perfectly what would happen at both ends of the call, if Jeremy is innocent.
Nevill is using the phone call to Jeremy to stall Sheila. Whether it was wise of him to do so is another matter and not important to the validity of the scenario.
Jeremy eventually awakens to the phone ringing, but understandably won't remember much about it. As soon as he answers, Sheila runs upstairs and Nevill follows her, terminating the call.
Jeremy claims what he claims about what Nevill said to him, but I don't consider it reliable because, if he is telling the truth, he was woken in the early hours of the morning and we can't expect reliability under those circumstances. He will have been confused and will have fumbled about for some minutes while deciding what to do. If he is innocent, then he is simply telling himself what he thinks he heard, which is not necessarily what he heard but what he has convinced himself of what he heard, which may have been influenced by subsequent events.
If, on the other hand, Jeremy is guilty and has invented a phone call (as opposed to staging one - a different thing), then he has taken a very fundamental risk because he was not a telecoms engineer or employed in that field, so would not know for sure whether there would be a traceable record of a call at the exchange, and he had no ready means to research and ascertain the position without running the risk of creating a witness or paper trail.
If, instead, he staged the call in the belief that some sort of record of calls may be held at the exchange, then he has the problem of how to get back to Bourtree Cottage within a set time limit without being seen. If he is seen, by anyone, at any point, it's over (the exception being that if he is seen immediately outside the Cottage, he can explain that away).