Naturally, all people have Confident and Unconfident voice depending on the situation they are in but they often get used to apply one of which.
When you are aware of your confident voice and you use it, then it will be infectious. By this I mean, other people will feel your confidence and can easily have confidence on you.
Hear your confident voice, imagine when was the last time you use it.
Using our confident voice is so joyful, however, we should be aware of the difference between an aggressive voice and a confident one because the border line is very thin.
Another interesting point I would like to make is that when we use our confident voice, our personality, our thoughts and behaviours get changed.
My experience says just use confident voice while talking and you will be self-confident automatically.
I also experienced that using my confident voice I stammer much less.
Practice for one day only talking with your confident voice and see the results