'There were four separate phones at the farm, all on the same line: one in the kitchen, one in the office and another in Nevill and June’s bedroom. A cordless phone had been collected for repair two days earlier'.
From Woffindens article. This has been discussed before. A lot. I've always said Bamber removed the kitchen phone off it's hook upon entrance. To disable all phones. Then moved the bedroom phone downstairs to explain Neville's kitchen fight.
Scipio said the bedroom phone was already downstairs.
To believe CAL got the idea from you is optimistic. Didn't all big houses in the 80's have one phone line ? So I'm sure she could work it out herself.