Abstract

A telephone switching system is disclosed with facilities for processing interoffice callback information over a common channel interoffice signaling system (4) between program controlled call processors (21, 34) in call originating and terminating offices (1, 2) upon encountering a called station busy condition. The terminating office call processor (34) is equipped to examine the busy-idle status of the called station (5) by consulting its supervisory memory (46) and by directed called station line scanning and then to enable an automatic call retry only after it is determined that the called station has become idle. Each of the call processors (21, 34) includes an originating register-callback register (47) for storing callback information comprising the calling and called station identities originating office identity, a callback code dialed from a calling station (3), a mark to specify the callback mode of register operation, a line recheck time delay and a time limit on the callback service. Automatic callback is effected in one arrangement by transferring the callback information from the originating to terminating office, ascertaining when the called station becomes idle, and then making a terminating-to-originating office call in an attempt to complete call connections between the calling and called stations. Another arrangement bypasses the terminating-to-originating office call and retransfers the callback information to the originating office for enabling it to initiate the call retry.