You may have anxiety which peaks up in situations where you have to speak in public and over the phone. I have this too and I feel like it's us, anticipating a negative or unwelcoming reaction from the other person (s) about our stutter. Also, in my experience, it's because I'm so focused on sounding "normal" or fluent that I forget to slow it down and use techniques to help me. My advice for you would be to not put a lot of pressure on yourself for stuttering, breath more through the nose and mouth, and practice picking up the phone in your spare time. Fun fact, I am also a college student!