A busy signal is a signal that indicates that the telephone number someone is trying to call cannot be reached because the phone is otherwise engaged, or because the circuits are busy, making the call impossible to complete. On many phones, the signal takes the form of a distinctive tone, but it can also be visual, as in the case of a phone designed for deaf or hearing-impaired users. Many people are familiar with the sound, as it differs markedly from the dial tone, the tone that indicates that the phone line is open and ready to be dialed.
In most cases, a busy signal simply indicates that the person being called is on the phone, or that the phone is off the hook. In these instances, the signal indicates that there is no technical obstacle to placing the call, but that the call cannot be completed because the phone is in use. In some cases, the caller can ask an operator to break in, if there is an emergency, in which case the operator will interrupt the call and ask if the subject wishes to take the other call.
In other cases, a signal means that the circuits are all tied up, so the phone call cannot go through. This sound is usually different from a regular busy signal; it is sometimes called a “fast busy,” because the tones tend to be more rapid, and it is also known as a reorder signal. This is common after a natural disaster, due to combined damage to the phone lines and a high call volume.
In both cases, callers must wait and try their call again if they want to get through. People who get a reorder tone or a message that “all circuits are busy,” may want to wait several hours, until the peak demand passes and the phone company has a chance to get more circuits operating. Some companies also offer a ringback service, which will try the number until it goes through and then cause the caller's phone to ring, connecting him with the person he wanted to call.
A busy signal is also sometimes referred to as a busy tone or an engaged tone. The sound of this signal is not universal across the world, so if someone experience problems using a phone internationally, he may want to ask a local what the different tones mean. Sometimes, businesses with their own phone exchanges also have in-house signals and reorder tones that differ from those used by the phone company, which is something people should be aware of if they make frequent calls to big companies.