I don't know where some of you work. I use my cell phone all the time to both recieve and place calls to other attorneys, the court, my friends, my co-workers, etc. My sister works in a bank, and we call each other, no issue. Frankly, there is such a blend in many people's lives between work time and non work time that if anyone ever said 'don't use your cell phone during business hours', I would laugh. I use my cell phone during business hours, to make and receive business calls. I use my desk phone to make and receive personal calls. It is a phone, a way to get in touch with me. I can easily program people's numbers into my cell phone, I can't do that with work.
Just last week, I used my cell phone, while sitting at my desk, to call another attorney on HIS cell phone, while he was sitting at his desk. Course, during the call, both of us had our office phone ring. LOL.
BUT, having said that, I don't have an offensive message on my VM, and do have certain ring tones programmed for certain people. None of them contain any vulgarity.
I would just say to her, "Did you know that every time your phone rings I hear the "F" word? Probably not very office appropriate."
That is kind of the beginning and end of it, IMO. You aren't her boss, she isn't an employee, and could you possibly sever a relationship that the company might not love you for?