Text messaging, or texting is a colloquial term referring to the exchange of brief written messages between mobile phones, over cellular networks. While the term most often refers to messages sent using the Short Message Service (SMS), it has been extended to include messages containing image, video, and sound content, such as MMS messages. Individual messages are referred to as "text messages" or "texts". The most common application of the service is person-to-person messaging, but text messages are also used to interact with automated systems, such as ordering products and services for mobile phones, or participating in contests. Advertisers and service providers use texts to notify mobile phone users about promotions, payment due dates, and other notifications that were previously sent by post or left as voicemail. There are internet services available that allow users to send text messages free of direct charge to the sender.