I told her if it's that bad she should call 911.
In fact, you need to remember: There is almost NO emergency that REALLY needs you to answer then. If someone is at death's door, and you have 1 hour to get there before they die--maybe.
But if she sprains her ankle at the mall and can't drive home, some kind person at the mall will help her. At *least* until you are free and can call her back, in 1.5 hours. And if it's bad enough that medical attention will make a difference, then, in the words of SOMEONE wise, "Call 911."
I think that the reason she's calling you at 8pm, or in the evening, is that this is a normal time to connect with people, and you're the person she wants to connect with. (Does she have no family living with her, I wonder?)
She'll know you're screening her calls no matter what you do in the future w/ technology, but it might be good to fix it so that it doesn't automatically do that.
And I wouldn't check the phone at all--just set it so ALL calls go to the answering function.
And you might make it a point to call her back at a set time, and then say, "I want to talk to you each night, at 9:30--how does that fit your schedule?"
In other words, focus on a positive, what you DO want ("I want to speak with you at X time") and not on what you DON'T want ("I don't want to speak with you at 8pm").