In order to make optimum use of the available transmission channels in an information transmission system, such as a trunked mobile ratio transmission system, where a number of uncoordinated users may wish to signal to a central control point in addition to using the channels for another purpose, the central control point monitors the channels and, when a channel is or becomes free, sends a command signal on that channel. When a user who is waiting to signal receives the command signal he chooses one of a predetermined number of immediately following time slots in an effectively random manner and sends his request in that slot in digital message form. The central control point sends an acknowledgement in the next time slot if and only if it receives the request in an error-free manner. If a user does not receive such an acknowledgement in response to the transmission of such a request he tries again when he receives the next command. The channel is used for other purposes, if necessary, at times other than those occupied by a command signal plus the subsequent predetermined number of time slots. If requests from more than one user should clash in the same time slot they will not be received by the control point in an error-free manner, this being determined by means of error check bits provided in the request message.