To get the Badge, you’ll need to walk for a Banana Telephone to spawn in the Lobby. To find out if one has spawned, you’ll hear the ringing of the Telephone. If you do hear a Ringing the Telephone will spawn at this Spot. Now what you need to do is basically go close to the Banana Telephone. Once you are, you’ll start seeing numbers around it. 
There will be a Total of 10 Digits which form a Code, Starting from the First Yellow Digit. It is suggested to write these Digits down after getting your 10 Digit Code, Click on the “Codes” Icon and enter your 10 Digit Code. 
After entering your 10 Digit Code, you’ll get the Badge and Receive the Evil’s Voice Banana Skin. The Banana Telephone spawns RARELY. The 10 Digit Code that the Banana Telephone gives is different every time it spawns. If a Banana Telephone Spawns and someone redeems the 10 Digit Code BEFORE you redeem it, the Code won’t work for you and will be Expired. 
So you’ll need to wait for the Banana: Telephone to spawn again and be the First person to check the 10 Digit Code and enter it to get the Badge. Also, the Banana Telephone spawns once the Current Round Ends. 
So basically, you need to be as quick as you can to get the 10 Digit Code and enter it on the Codes Menu to get the Badge and Skin.