A two-way wireless selective call communication system (20) using a synchronous time division multiplex communication protocol transmits messages divided into a plurality of data units (120, 126, and 128) from a plurality of selective call transceivers (32) to a paging terminal (33) during time slots (78), the number of data units and the time slots are set by the paging terminal controller (26) based on information, including size of the message information, received in an unscheduled transmission from each of the plurality of selective call transceivers. The paging terminal transmits to a selective call transceiver a multi-bit acknowledgment message having a one-bit acknowledgment for each of the plurality of data units. A selective call transceiver re-transmits data units not recovered by the terminal based upon content of the acknowledgment message. More than one selective call transceiver transmit during a one time slot set by the terminal.