Ok, you may hate this idea, but one thing you could do, outside of your job, is go somewhere and talk to strangers. Just, hello, how's your day going? Or whatever you would like. It sucks, and if you have anxiety like me that can make it worse. It's to make you get used to it and help you build confidence. It's just strangers who you may never see again, it doesn't matter what they think, it matters what you think. I used to be absolutely horrible on the phone. Then my job in customer service actually helped me, it forced me to talk to people, which I hated, and now I'm more fluent and generally better at it.