A distorted mind equals a more distorted view of the real facts in this case.
Adam,,consider that Neville could have used the phone in his office in which to ring Jeremy. If Sheila had been downstairs at the time,,which she could well have been,allowing Neville that short time to ring Jeremy,,,but,Sheila being downstairs,would have heard any conversation on the kitchen phone and belted upstairs to Neville pointing the rifle.
Neville would have replaced the office phone back on its cradle,but the kitchen one would have still been off its hook,so therefore Jeremy wouldn't have got an answer. Simple ? The engaged tone when Jeremy tried ringing again.