Overview: MS Teams calls "delegates" as someone that can make and receive calls on your behalf. "You can pick someone in Teams to be your delegate—when you add a delegate, you’re essentially sharing your phone line with them, so they can see and share all of your calls."

Please note: Configuring Delegates is ONLY accessible on the Microsoft Teams DESKTOP client for Windows and Mac

Step 1: Set up the process to "Share your line with a Delegate"

Step 2: Delegate confirmation (in Microsoft Teams)

Executive User (username) added you as a delegate.
Now you can:
Make calls on their behalf
Receive calls on their behalf
Change their call and delegate settings

Screenshot from MS Teams: 

Step 3: Make and Recieve Calls as a Delegate 

How to: Put calls on Hold as a Delegate 

How To: Change your boss's call and delegate settings

Use Case: Take and make calls for an Executive to a Receptionist relationship.

1st example: Warm Transfer (CONSULT) Call for Executive when answered by Receptionist.

Collaborate with the Executive via CONSULT option in MS Teams. The Receptionist can message or call the Executive

2nd Example: PARK and RETRIEVE Call for Executive when answered by Receptionist.