Call waiting is usually enabled by your phone provider by default. You shouldn’t have to enable it manually unless it’s been disabled for some reason. Depending on the model of your Android, the menu options may vary. Basically, you’ll be opening the Phone app’s Settings menu to find your call options.
You may have to scroll down to find this option.
Your settings will automatically update, and you’ll now be notified of incoming calls when on an existing call.
