Whereas my mother's dementia meant that not only was she unable to use the phone for the last two years of her life but that she was also scared and intimidated by it. I therefore stopped trying to speak with her on the phone as it was far too upsetting for her.
I also worked with people with dementia who no longer had any concept of time of day. They would telephone people at any and all times be that 3.00pm or 3.00am.
Everyone's experience of dementia is very individual. There are reasons why restricting access to a telephone is maybe not as cruel as it may sound without knowing the full context.