Everything you do requires energy, and the more strenuous the activity, the more calories you burn. So we know straight off that talking isn’t going to shed those pounds. How many calories do you burn by talking? It turns out you burn barely more than just by being alive, as given by your basal metabolic rate.
When we speak, we expel air through our voice box, making our vocal chords vibrate and produce sound of a certain frequency. This frequency is determined by the width of the vocal chords, which control the wavelength of the standing wave producing the note. We can change this note by changing the distance between the vocal chords. However, varying the distance between the vocal chords is negligible with respect to calorie burning, compared to producing the air pressure required to produce the note.
If you talk louder, faster, or move to singing or shouting, you will burn more calories. This is because you will be using more air, and so your heart will have to work harder to keep oxygen flowing through your bloodstream. An increase in heart rate is beneficial to your cardiovascular system and will increase fitness. This is what makes aerobic exercise a healthy pastime. But the amount of calories you burn by talking on the phone will not compete with cycling, running or swimming.