Yikes, I'm not a phone person either, & mostly not verbal. One way might be to write a brief outline about what you expect to talk about with your friend so it's not extemporaneous. That should make it easier. I guess if you feel comfortable enough you could also tell him, from the start, you're phone nervous. You said he's very nice so he should be understanding. Good luck...hope it's not a job interview or dating related.
Oh, you could reduce or eliminate all extraneous stimuli such as the radio or anything visually distracting. Not quite the same, but when I have a take-home exam I often light a candle, maybe make some tea, which makes me be not so nervous. Outline helps, even rehearsing some. Good luck.
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The ones who say “You can’t” and “You won’t” are probably the ones scared that you will. - Unknown