Speaking on the phone used to be impossible for me as a teen, but no longer is as an adult for some reason. What I would recommend based on what I know now, and hindsight, is that disclosing your stutter can help a lot with relieving tension buildup, anticipation, and speech blocks, whether that's bringing it up with colleagues or supervisors (depending on your comfort level and how accepting you feel they'll be), or mentioning it to the person you're speaking with over the walkie-talkie. I've found most adults especially in a workplace are very mature and gracious about stuttering. It might also help to focus on the advantage of being unseen. The person can't see you forcing a word/blinking hard and all that – they may hear stammering or a 6-second random pause, but since you're physically not visible, you could think about using that to your advantage somehow.