Communication

 One example out of fifteen in "Communication" in Chapter 21:

 Written Communication:

• Handwritten or typed notes left at crime scenes as a form of communication or intimidation
• Anonymous letters sent to victims, law enforcement, or media to claim responsibility or issue threats
• Correspondence between criminals or with accomplices through written letters or notes
• Handwritten notes or letters used to convey messages or threats
• Graffiti or symbols left at crime scenes as a form of communication or identification
• Posters, flyers, or other printed materials used for criminal purposes or to send a message
• Messages or markings left on victims or objects as a signature or form of communication

See the book for the other communication categories! It's not only mobile phones...