Possibly need a bit more contextual information but . . .
I find a really good quality headset helps me pick up subtle information you can miss in a speaker and excludes any background distraction noise (for me or the listener)
Also there is something about standing up that changes your tone, I use a stand up desk and always stand up for important calls
Feeling more confidence come down to all sorts of stuff around prep, practice, visioning etc. Before tricky calls I often rehearse with someone I trust and try out a few directions it might go in