Mum just called. The reason she hadn't answered my call was that she hadn't heard the telephone ring. She hadn't heard it ring on several occasions when we were going to take her to the pub on Boxing Day. Either her phone's not ringing loudly enough or (more likely) she had the TV on too loud for her to hear it :)
Dad's not in hospital, nor is he any worse. Last night's delirium was almost certainly due to a fever spike, but he's breathing fairly well again and is eating a little and drinking, so there's no grave cause for concern.
Prayers, good thoughts and suchlike for him would still be much appreciated, though.