The feds love burner phones used by stupid criminals. If you are compelled to use a burner phone, use it far away from any of your other devices, use it only once, and when you are finished take out the battery, break it up into little pieces, and spread the pieces in different, remote places.
I guess it's because I have done work in this field that I hold people in contempt who domn't understand the level of surveillance they are under.