Your child becomes upset and you are out of sight. While remaining out of sight, you call to your child by saying their name, soothing them, and/or reassuring. Your child recognizes your voice and calms.
You leave the room to get a glass of water. Your child becomes upset. Hearing your child, you call their name and reassure, "I'm coming back!". Your child hears your voice, recognizes it, and calms.
Take it one step further by incorporating a language-expansion activity! After your child calms, you can label those emotions.