Tie-Lines were a really, really big deal in the 1960s through the 1980s. Every major company had a large staff dedicated to managing the corporate tie-lines.
A tie-line was a direct dedicated connection between two corporate locations. If a company had a headquarters in Los Angeles and a branch office in San-Francisco it would be tie-lines that connected them together. There were two benefits to tie-lines:
- They were usually much cheaper than paying long-distance.
- You could dial an extension number at the destination end.
Without a tie line, somebody in the LA office wanting to reach a co-worker in the SF office would need to dial long distance to the main number in SF. Then, a switchboard operator would connect your call to the extension number of the SF co-corker. It was slow and expensive.
If the company had enough calls between LA and SF they could justify paying the phone company for a dedicated line. The circuit would be there, permanently wired, 24 hours a day. A tie-line could originate calls on either end to the other. So at SF the line would give dial tone from the PBX in LA. And, at the LA end you would hear dial tone from the SF PBX. The cost for a line like this would typically be about $1,000 a month.
Employees accessing tie lines was an art form, especially in a large company. You would need to look at a map of corporate offices, find your office (LA), and then look for the access code to the office you wanted to call (SF). It might be a code such as 81. So you would dial 81, wait for SF dial tone, and then dial the extension number in SF. If your company had another branch office in San Jose (SJ) there would be tie-lines between SF and SJ, but probably not between SJ and LA. Thus, a person in LA who wanted to call somebody in SJ would dial 81 to get SF dial tone, and then perhaps dial 72 to get SJ dial tone, and then dial the extension number.
A large corporation would have dozens of main offices and perhaps 100 or more branch offices. The tie-line map would be very large and complicated, and you needed to navigate this to call an associate.
A telecommunications staffer would be looking at utilization and long distance bills and would decide how many tie-lines were needed between which locations. And, as they were expensive, they needed to be heavily utilized and so they were often busy. It was not uncommon to have this happen: Dial 88 to get to SF, dial 72 to get to SJ, dial 71 to get to Santa Cruz and get a busy signal indicating that this line was busy.
Companies would spend hundreds of thousands of dollars monthly on their tie-line networks, and would often have a staff of a dozen or more people designing the circuits, and the phone companies would have a support staff to help the company staff.
By the 1990s, tie-lines started to fade away because long-distance rates dropped while dedicated circuit pricing increased, and also automated-attendants allowed extension numbers to be directly dialed at remote offices.