It was used to signal the status of Trunk lines, specifically if they were in use or open/available. By blowing a 2600 tone at the right time, you could seize t

It was used to signal the status of Trunk lines, specifically if they were in use or open/available. By blowing a 2600 tone at the right time, you could seize the trunk and reroute the call. I miss those days. Crawling in dumpsters at the local CO, staying up until 5a, on a school night, because you'd just discovered something really, really cool.