I'm sorry to hear about this but you are not alone in receiving out of hours and/or repetitive phone calls. If you do a search you will find lots of threads about this. My mother mostly did the repeating calls, only a few in the middle of the night, but she did get her days and nights turned round. It was distressing to be woken out of a sound sleep and then have her be oblivious and chattering away about nothing of import, when you were expecting something awful. We had to get voice mail (I couldn't stand to listen to her messages on the answering machine) and caller ID so I could choose when to answer the phone. I went through a period where I would cry when the phone rang. It's very stressful.
You may need to consider turning the phone off at night, or having a dedicated number for your mother to ring that, when you don't answer it, will go to voicemail/answerphone with a message like "Hi, Mum, I can't talk right now but I will ring you soon!" or something reassuring to that effect.
Here is a thread to get you started (with links to others) if you want to have a read here on TP about others' experiences with the phone calls:
http://forum.alzheimers.org.uk/showthread.php?89569-Phone-Calls
As I understand it, it's common for people with dementia to lose their sense of time and this can happen at any stage, even early on. I think it's related to memory loss in general but am not sure. Have you read any of the factsheets on the Alzheimer Society's website? Many of them are quite good. Here is one about general memory loss:
https://www.alzheimers.org.uk/site/scripts/documents_info.php?documentID=123
My only other thought is to ask what sort of routines and support your mother has. Does she have daily carers coming in, does she go to day care or a lunch club, that sort of thing, that would help give her social contact and structure?