Data communication system

The present invention relates to systems for communicating data. In particular, systems for communicating data to and from mobile devices are described. One system involves the use of a server to store the association between a shortcode and the data. A user device can be used to transmit the shortcode to a second user device. The second user device can then access the association at the server to access the data. The shortcode is transmitted via an audibly unique signal between the two devices. Further devices, systems, and methods for redeeming vouchers, transferring money, unlocking content on a device, and authenticating transactions all using audio are also disclosed. A method for managing data communication on a mobile device is further disclosed. In addition, a system for asynchronous transmission of the data is disclosed by use of reserving a shortcode at a server before the shortcode is associated with data.