Why does the text message remind the caller that he is in service after the call cannot be connected?

Call reminder service. After the phone can't be connected, the short message reminds the caller that the friend is in service. This is because the call reminder service has been opened. At this point, the friend is in the service state. Telephone is a modern communication tool, which is convenient to contact with work and handle affairs. Voice communication can be divided into two forms: consumption of traffic and consumption of telephone charges.