The number format in the United States for accessing an interexchange carrier (IXC) for casual callers without a presubscription agreement with an IXC and for dial-around purposes in the event that the presubscribed carrier is suffering from network congestion or a failure.The format originally was 10XXX, but competition encouraged the formation of so many IXCs that the format had to be expanded. See also IXC and presubscription.