Talking of the phones it is quite plausible that if Jeremy committed the crime, he would have moved the bedroom phone downstairs afterwards. To explain why Neville was in the kitchen.
It may also explain why Neville went downstairs. Upon entry the kitchen phone may have been taken off the hook. This disabled all other phones in the house as there was only one line.
Dont know why the kitchen phone should be unplugged & in another part of the kitchen if it was in perfect working order.