What method of call sign identification is required for a station transmitting phone signals?

When you are transmitting phone (voice) signals, the FCC requires you to periodically identify your station. You might naturally assume that a phone transmission must be identified exclusively using a phone emission, but the rules actually permit identification by either phone or CW (Morse code). CW identification is universally accepted across amateur bands and modes. This makes it legal and valid to use Morse code to identify your station even when your primary transmission is voice. In fact, many repeaters use automated CW to identify themselves while users are transmitting voice.