When I've had issues with my employers, I've always disliked explaining it over the phone. I think having a formal, face-to-face meeting is far more important.
From my experience, no matter how much you would prefer them to tell you then and there, their issues, pressuring/bullying them into telling you does not help the situation.
In my last job, I went off sick for a week, and I didn't handle it properly (to make a long story short). The owner was getting really pissed off why I wouldn't tell him why I was ill (none of his business), and why I apparently had said things to staff. He was getting very upset at me over the telephone, and it was basically making me feel even more not well, and not wanting to talk to him, or go back to work after my sick-leave.
Needless to say, there was fault on both sides, but if he had approached me in a calm, polite matter, in person, then the missunderstandings would have been greatly minimised. Eventually we did have a catch-up after my return, and while it was very good to talk to him, I just felt he dismissed everything I had to say.
My point of my story is, if you want this employee to stay, be calm, polite, and receptive to everything they have to say. Even if you think their complaints are not justified, record them, and have a think about what you can do to help this employee. It could be a simple misunderstanding.
Good luck, and let us know how you get on tomorrow.
