Editing Blue box (section)

===Long-distance direct dialing===
To address this need, the Bell System adopted a second system on the circuits that connected the exchanges. When the user dialed a long-distance number, indicated in North America by dialing a "1" at the beginning of the number, the call was switched to a separate system known as a "[[Class-4 telephone switch|tandem]]". The tandem would then buffer the remaining digits and decode the number to see which remote exchange was being dialed, generally using the [[area code]] for this purpose. They would then look for a free trunk line between the two exchanges; if none were available the tandem would play the "fast busy" reorder signal to tell the user to try again later.{{sfn|Rosenbaum|1971}}

The basic protocol for finding a free line worked by playing a 2600{{nbs}}Hz tone into the line whenever it was not being used. The tandems at both ends of a given trunk line did this. When the tandem determined which remote exchange was being called it scanned the trunk lines between the two exchanges looking for the tone. When it heard the tone on one of the lines, it knew that line was free to use. They would then select that line and drop the 2600{{nbs}}Hz tone from their end. The remote tandem would hear the tone stop, drop their tone, and then play a ''supervision flash'', making a "ka-cheep" sound, to indicate they had noticed the signal. The line was now free on both ends to connect a call.{{sfn|Rosenbaum|1971}}

Pulse dialing still had the problem that sending the dialed number to the remote exchange would not work due to the capacitance of the network. The tandems solved this by buffering the phone number and then converting each digit into a series of two tones, the [[multi-frequency signaling]] system, or "MF". Once the local tandem had found a free line and connected to it, it then relayed the rest of the phone number over the line using the tone dialing method. The remote tandem then decoded the tones and turned them back into pulses on the local exchange. To indicate the start and end of a series of MF digits, special MF tones, KP and ST, were used.{{sfn|Rosenbaum|1971}}

When the call was finished and one of the parties hung up the phone, their exchange would notice the change in voltage and begin playing the 2600{{nbs}}Hz tone into the trunk line. The other end of the connection would respond to the tone by causing their local call to hang up as well, and then began playing the tone into their end as before, to mark the line as free at both ends.{{sfn|Rosenbaum|1971}}