If he planned it, he planned it. Tracing of phone calls is a very obvious point. If what Rob said on the other thread is true and the line could only be closed at the caller end, then Jeremy can't stage the call from Nevill. It's impossible. Furthermore, we know that Jeremy did later on make calls from Bourtree Cottage.
Thus, what the guilt camp are telling us is that Jeremy has decided to wing it and invent a call from Nevill and hope either that the police don't check with BT or there is no practicable way of establishing the position. I find that rather unlikely. The pro-guilt camp agree with me, which is why they explain it as Jeremy's arrogance. Notice what Jane is saying. Think about it. Jane is tacitly conceding the point, but saying: 'Oh, but Jeremy is arrogant and would have chanced it'. Jeremy's supposed 'arrogance' has become the deus ex machina of the pro-guilt camp as they confront the practical and logical difficulties of the prosecution scenario.
My own belief is that, if Jeremy did this, the phone calls were an unplanned aspect and thought up on the hoof due to Nevill ending up in the kitchen. This is based on the simple logical observation that Jeremy could not - and would not - plan for Nevill to be in the kitchen. Even if Jeremy knew that Nevill would be sleeping downstairs, he would kill Nevill where he found him, for two reasons: (i). he needs to make it look like Sheila has run amok; and (ii). he needs to kill Nevill anyway. If Nevill's body is found in such a way that suggests he was struggling with Sheila in the kitchen, Jeremy may have decided he needed to give himself what he considered to be the extra insurance of an 'alibi', otherwise investigators might well ask how it came to be that Nevill is running through the house away from a slight, weak woman (of course, there are rational explanations for this anyway - for one thing, she has a loaded rifle - but we need not go into that now, and the point is that Jeremy will not have thought about it that way).
In that scenario, Jeremy may well have decided to wing it, but the guilt camp also want us to believe that Jeremy would tell the police to make inquiries with BT. Why would Jeremy do that?
My goodness, this Jeremy was a helpful, public-spirited chap wasn't he! He commits mass murder and helps the police catch him. I suppose it was only fair, as Taff did give him a sporting chance at the start.