An equal possibility is just because Ann didn't see one, isn't necessarily indicative of there not being one.
It’s quite possible that Ann Eaton wasn’t aware he had an answer machine. She says in her statement that the one phone in the cottage was downstairs and had a long lead, so he could have left the telephone on any table in either the lounge or dining room depending on where he last used it.
The answer machine can’t be moved far from the telephone socket, but the leads are are long enough to place it underneath a table if you lacked room or something. Holly said that Ann never mentioned the answer machine, but maybe she didn’t notice it or even see it. She may not have even known what it was unless she studied it.
It’s irrelevant whether he had one before he murdered his family or not; it doesn’t change anything. And he could have said he hadn’t had it switched on when he went to bed, so it’s all pointless really.
But considering he liked the latest gadgets of the time and wanted teletext television etc when he moved into the cottage, I’d say he bought one before he killed everyone as he was working on the farm and would have needed one for his illegal drug dealing.