Base-station apparatus, communication system, and communication method

A base-station apparatus includes: a communicator that performs wireless communication; and a controller. During communication with at least a first terminal apparatus, when a communication volume used for communication that is newly stated with a second terminal apparatus becomes smaller than or equal to an un-used communication volume, the controller transmits, to a target terminal apparatus that is one of the first and second terminal apparatuses, a communication instruction for communication with a second base-station apparatus having a coverage range in which the target terminal apparatus is located, and when a communication volume used for communication with the target terminal apparatus exceeds the un-used communication volume, the controller transmits a callback instruction for callback from the second base-station apparatus to the base-station apparatus to the target terminal apparatus or the second base-station apparatus.