Although the Easter egg was discovered on Android 6.0 Marshmallow’s stock dialer, it still works on modern Android phones, provided you use the Phone by Google app. To activate the Easter egg:

Open the Phone by Google dialer on your device.
Punch in 0118 999 881 999 119 725 3 like a phone number.
After entering the “3” the Call button will flash blue and red, like a police car. Your phone should also vibrate rapidly.
Remember that the feature only works on the Phone by Google app or the stock Android dialer.