This isn’t a permanent solution but I sometimes have a whiskey or a beer before an important phone call. I know that leaning on booze for a solution isn’t healthy but it really helps me, to the point that sometimes people who I’ve met at a bar or a party sometimes don’t even realize I have a stutter at all. Of course, don’t do this if you’re not a regular drinker or you think it will fuck with your ability to do the interview. As others have said, walking helps too. I also like to find things I can do with my hands, as fiddling with stuff takes my mind off of speaking and makes me more fluent. Good luck, mate. I know how stressful this shit can be.