How to handle passive/aggressive co-worker?

Don't let her behavior bother you, she is just trying to irritate you. If I were you I would continue answering her ringing phone, don't mention it to your boss, and do it well. The last time I answered another persons ringing telephone, a task that wasn't necessarily my job but I did it glady, I got nominated for an award. It was actually one of the best things that ever happened to me. My point is just ignore her behavior, and maybe not immediately, but if she doesn't straighten up her act she will cause her own demise (not that you want that to happen but she will straighten up).