Ugh I've experienced this before, didn't understand how I could be fine for months saying an introduction on the phone not being able to at all. I did these 2 little hacks that helped me through it though. One of them was that I would write my intro on a little piece of paper that I always have on me, and when I answered the phone I would read my intro rather than organically saying it. Another was to have a mirror near the phone, just a little one, looking in the mirror as you talk on the phone works for some reason. I started doing these tricks after I saw some other stutters on tiktok use them. Keep in mind that you don't have to speak immediately after picking up the phone, give yourself a beat to take a breath and then speak. Try changing up your intro, cutting down the length of it or changing up how you say it. You will get back into the groove of it eventually!