Advice for phone confidence?

I work in an office and a large part of my job is to make and receive calls.

When I’m working from home, I’m super confident on the phone and the conversations go so smoothly. When I’m working in the office (which, by the way, is mostly dead silent), I find that my voice goes croaky and my calls always sound worse – because I know people are listening.

Have you got any advice for overcoming lack of phone confidence / not caring what people think?