When the call began to ring, the caller would hold the blue box speaker over the microphone in the handset and use the box to send the 2600 Hz tone (or 2600+2400 Hz on many international trunks followed by a 2400 Hz tone). The called office interpreted this tone as the caller hanging up before the call completed, disconnected the call, and began playing 2600 Hz to mark the line free. However, this did not disconnect the call at the caller's end, but instead would leave the caller on a live line that was connected via a long-distance trunk line to a target exchange.
When the call began to ring, the caller would hold the blue box speaker over the microphone in the handset and use the box to s,end the 2600 Hz tone (or 2600+2400 Hz on many international trunks followed by a 2400 Hz tone). The called office interpreted this tone as the caller hanging up before the call completed, disconnected the call and began playing 2600 Hz to mark the line free. However, this did not disconnect the call at the caller's end, but instead would leave the caller on a live line that was connected via a long-distance trunk line to a target exchange.