Yesterday my co-worker called me 3 times to see if I would trade shifts with her today. I was not available to take the calls, nor was I able to return the call until very late in the evening. She left a voicemail all 3 times, and in the last one all she said was "Thanks for calling me back" in a very hateful tone. Needless to say the last message really irked me, and if it hadn't been too late to call already, I probably wouldn't have returned her call because of her rudeness. My question is, if I see her today (our shifts would overlap if she shows up for work) how do I handle it? I feel I don't owe her an explanation, but do I say something about the nasty voicemail or just pretend like nothing happened?