Since it is over the phone, I wouldn't mention it unless they do. You don't know if they are at a point where they are comfortable with their stutter. If in person, you can try and read body language etc to see if they are a little frustrated with themselves. For me personally I don't mention my stutter first. If I'm having one of those days I will eventually mention it at some point in the conversation because I feel bad for thinking I'm wasting their time. One sentence I rarely ever stutter on is, "sorry, I have a stutter"