"Yes, I realize that issues do come up outside normal hours. That's why you can call our 24-hour line and talk to someone any time, day or night. I'm only here from eight-thirty to five on weekdays, though, so if it's something I personally have to do, I will try to get it done as soon as possible when I'm next at work."
If she tries to tell you you're required to answer email / phone calls at all hours, repeat: "Oh, I'm afraid that's not possible for me. You can call the main line, though, and talk to someone else who can try to help you!"