Of how the phones worked at WHF
Seeing that you know everything.
There was a single line in to WHF, all phones were usually connected to the single line (although obviously not on the night of the murders).
They could each be used to dial out, but not at the same time. If a call was in progress on one phone, then another phone could be picked up and would be connected to the conversation.
If a phone was off the hook and the dialing tone had ceased, then all other phones in the house would be blocked from making or receiving calls.
There was no extension system, you could not dial a phone in the house, from another phone in the house.