My coworker keeps calling me during non-work hours

This is weird, you guys.

I have this coworker. She and I are definitely not friends, so there is no real reason for her to be calling me for any non-work purpose. Our working dynamic is polite but distant at best, with some underlying tension. She can be passive aggressive and lazy, and I often resent the fact that I work a lot harder than her. She tries to make me look bad to our boss by nitpicking my work and cc'ing our boss when she thinks I've made a mistake. Which tends to backfire because it's actually her own mistake she's calling out and when I point that out she doesn't respond. But besides the passive aggressive emails, I try to keep it mostly professional and I say hello when I see her in the office. Sometimes she'll have a brief conversation with me, but it's never much.

Recently, on multiple occassions, she has called me at very odd times of the day, when I am not on the clock. We have work phones from our company, but she is calling my personal cell instead, at like 9pm on a weekday or even 6am on a weekend. I've never answered it. She doesn't leave a voicemail or send a text, and when work hours come around, she never follows up with anything. She just goes about her day and never mentions it. But then it happens again at another weird hour of the day.

What is she doing? What should I do?