I speak better on the phone vs face to face especially with unknown persons . I think the reason is that i know probably i will never met that person or speak to it again ,so i don't really care if i stutter and i embarrass myself . But when i know i have to make a phone call i get nervous and the best way to deal with it is to make it right away .The more i delay it the more hard it's gonna be .Another trick is to try to distract yourself and not thinking that you are gonna stutter .