This article only related to Yealink IP phones supporting
When you receive your new Yealink phone from Nuacom
(that runs Teams and Nuacom in parallel),
You'll need to enable the toggle to switch between the two platforms.
In Yealink terms, it is called a "Quick ball".
Short instruction:
Go to Settings-->>Device settings-->>Hybrid mode-->>Quick ball-->>Slide it to enable.
Long instruction:
Step 1. When the phone boots up and displays Teams sign in "menu" press the "Settings sign" 
Button in the upper right corner
Step 2. In the Settings menu, navigate to the "Device Settings" option and press the "OK" button to select it.
Step 3. Scroll down to "Hybrid Mode " option and press the "OK" button to select it.
Step 4.In the Hybrid Mode Menu navigate to the "Quick Ball" option and press "OK" to toggle it on.
Step 5. On your Yealink MP54 Teams phone, locate the mode switch button, a circular icon with the letters SIP on it. The button is typically located on to the bottom left corner of the screen.
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Press the mode switch button once to switch to the SIP mode. The button will now display the letters TEAMS instead of SIP.
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To switch back to the TEAMS mode, press the mode switch button again. The button will display SIP again.