There is no reason for stuttering more on phone than in a real conversation, except for psychological reasons. So, you stutter more on phone (and I do that too!) just because it's a unusual situation. So practice it as much as you can. I started calling my friends instead of texting, for example. That is just a little bit outside of my comfort zone, and it helped me gain confidence in my phoning skills. The usual advice applies also: unless you are being interviewed for an actor role, people will care about what you said, as opposed to how you say it. It's really surprising how people can completely forget about speaking with someone who stutters, or they just don't care as long as your message is clear. Also don't think about your phone call before, it's not helping and it can be counterproductive, actually. If you really need to think about it, envision yourself speaking slowly, clearly and fluently. Mirror neurons can do wonders!