I’m sorry this is upsetting for you OP.
To play devil’s advocate though-
People with end stage dementia are likely to struggle with phone calls or people just turning up at windows as others have suggested.
Visiting in person may go well as seeing you in person gives lots of visual cues, but this doesn’t tell you much about other forms of contact.
It may well be the case that phone calls are unsettling and therefore not particularly useful for the majority of residents.
While it’s upsetting for you, you might need to ask yourself who the calls would really benefit. If it’s predominantly you, rather than your relative, you might need to accept the sad loss of your contact with her just now, and rest assured that if this really is end stage dementia, she is unlikely to be experiencing the passage of time in the same way you are.
That said, go with your gut and if you really think it’s of benefit to her, find a way forward with staff, while hearing them out on their view.