Most of your post is totally innacurate,but never mind.
What is inaccurate? The statements of those involved with servicing the phones and using them is what I went by.
Here is the phone company agent about the phones:


Here is the person who took the cordless phone away because they didn't want it anymore and he took the 2 way splitter as well so it was no longer possible to plug 2 phones in the kitchen, he noted the cream rotary phone normally in the bedroom was in the kitchen at the time he removed the cordless phone so it was moved to the kitchen prior to the murders


Here is where it is recounted that Jeremy lied and said the hidden phone was an extra phone though they never had an extra phone it was the phone normally kept in the kitchen and how he had previously claimed the bedroom phone was moved to the kitchen because the kitchen phone was broken

Barbara wilson

So what seems rather obvious is that in anticipation of the murders Jeremy moved the bedroom phone to the kitchen and lied and stated the kitchen phone was broken and he hid it.
He continued to tell that lie to people after the murders including to Jean. Upon Jean finding the kitchen phone which he had hidden she tested it and found out it worked and then asked him why it was hidden. He told her it was just a spare phone not to worry about it. But she later thought about it and realized it was the kitchen phone which he had told her was broken.
Barbara also recognized it as the kitchen phone and she also tested it and found it working and chose to use it instead of the dial phone for the remainder of the time she worked at WHF.