Abstract: A network operations center transmits a data message to a wireless mobile unit and waits for a data acknowledgment message. If no acknowledgment is received within a specified time, the network operations center sends a probe message to attempt to locate the mobile unit and waits for a probe acknowledgment message. If still no acknowledgment, the network operations center marks the data message as undelivered and stores it for future delivery. If a mobile unit receives a probe message while its transmitter is powered off, it displays an indication to the subscriber that there is a message waiting to be delivered. The subscriber can then dial into the network operations center to retrieve the message. Or, when the transmitter of the mobile unit is powered back on, the mobile unit sends a registration message to the network operations center; and upon receiving the registration message, the network operations center automatically re-transmits the undelivered data message to the mobile unit.