If you've ever learned guitar, you know that bar chords suck in the beginning. The only way to get better at them is to just keep doing them. Your finger isn't strong enough, notes are rigging out dead, but you still practice it every day. Then one day, it rings out perfectly. When you try it again, it doesn't. It's back to sounding like shit, but you heard it! So you keep working at it. Eventually you can play them semi-regularly so you start incorporating them into songs. Sometimes they don't ring out in the song properly, and sometimes they do. Half the time people don't even notice. The same is with stuttering. You have to keep practicing. You have to keep doing it. So how do you get better at phone calls? Make some phone calls. Next time you order some pizza, don't do it online. Call it in, and write out a script. "Hi, I'd like to make an order to go. I'd like a large pepperoni pizza. My name is (name). My address is (address). My number is (number)." Do the same with stores. Randomly call and ask if they have something in stock, even if you have no interest in it. Everyone is looking for a PS5 right now, ask for that. Unfortunately there is no special trick, you just gotta keep doing it. You'll have your good days and bad days, but you can do it. From, a stutterer who is on the phone 5 days a week for his job.