The science. Smiling raises the soft palate, opens the throat, and adds high-frequency overtones to your voice. Listeners reliably detect a "smiling voice" in blind audio tests. They rate that voice as warmer, more competent, and more honest.
Where to use it. Cold-call opener (first 7 seconds). Reframing an objection. The pause after price. Not during the close — there you want a flat, neutral, assumed-yes tonality.
The trick that actually works. Put a small mirror next to your monitor. You'll catch yourself going dead-faced 40 times a day. Each catch = one smile rep. Inside two weeks it's automatic.
Don't. Force a grinning, news-anchor smile. That bleeds through as fake. The smile you want is a slight one — the kind you'd give a friend you actually like.