Microsoft Teams - Private Conversations, Calling
In Chat, you can have private one-on-one or group conversations that are not in the public team channel.
Create a new chat Select Enter the names of the team members you want to chat with. Share files on a chat Select Select a file. You can add files from your computer or from your OneDrive for Business account. See your chat thread or shared chat files Select Chat, then select a person, persons, or bot under Favorites or Recent. Select Conversation to see your chat conversation thread. Select Files to see files shared during the chat. Turn a conversation into a call Select
to the left of Search box.
in the Type a new message box.
or 
to turn the chat into an audio call or video call.