We had a block of numbers: xxx-yyy-1000 to xxx-yyy-8999. Four digits were sufficient to call internally. No 9’s since we dialed 9 to get an outside line. 0 was reserved for the main switchboard.
As others describe, it’s been an evolutionary process.
Basically, every number in N. America was the 1+10 digits - 3 area code, 3 exchange, 4 local. That at least was standardized. (Whether that’s because Bell had a monopoly unless there was an existing local phone company, I don’t know)
I recall a small Canadian town many moons ago where you could just dial the 4 digits locally. It was big enough to have its own switch so that worked, but any external numbers dial 1+area code+7 digits. When things were replaced by digital switches, that option disappeared. Dial 7 digits. I’m told that recently (last decade or two) that province like many others went to 10 digit dialing to accommodate extra area codes.
At a certain point, instead of subdividing area codes and having half the population get the inconvenience of a new area code, we ended up with 10-digit dialing and overlapping area codes. So Toronto was 416. Then the area around Toronto was made 905, disrupting people. Then years later, Toronto proper got an additional area code 647 and everyone has to dial 10 digits. For long distance, prepend a “1”.
Some companies rented their entire phone systems from the phone company - so it was easy at that time to reserve an entire block, and each extension was also its own 10-digit number. This, however, could be expensive. Alternatively, there were internal PBX (Private Branch Exchange), the earliest form of what the OP asks. Actually, before that there were switchboards. Much like the original pre-switching days, a call would come in in a trunk and the company operator (often also the receptionist) would answer and then plug - and later just forward - the call to the correct extension. As usual, this mundane task was automated to a PBX - call in on the trunk, and either tell the company switchboard who you were looking for or dial the extension at the prompt.
Separate companies that sold PBX equipment found it easier to arrange for a set of trunk lines (each active call tied up a trunk line) and fan out to the extensions from there. Internally, just dial the extension. Extensions could be 2 digits or a lot more, depending on need. This avoided needing a block of numbers, something the main phone company would find easier to administer. There were several disadvantages - the number of trunk lines depended on the level of calls in and out. I worked at a place where the system would say “all circuits are busy - please try again later”. This became particularly a problem in the early days of dial-up modems. Phone conversations were usually a few minutes, especially back when long distance cost real money. Modem sessions could last an hour or more. (We found that one person was tying up his modem phone line all day being on-line by modem to his internet honey on the other side of the continent, thus using 1 of the company’s 5 outbound trunks.) the other disadvantage was dialing in - you could not just dial the full number, you had to dial the 7 /10 digits, then wait for the prompt to dial the extension. Not so bad for humans, but not easy for mechanized dialing. This is why the block number system became more convenient, and why often a business’s fax number was different from their switchboard number - it was a separate external line not inconvenienced by trunk capacity.
Of course, with digital switching and VoIP, the number really doesn’t matter. An area code once routed your call to a locality where the exchange 3 digits routed you to a local switch, where the 4 digit local told that switch which wire to connect. With digital, and especially with VoIP, the number lookup tells the system how to connect to your actual device(s) anywhere in the continent. There’s an new layer of abstraction between the physical connection and the number.
We have both systems. It depends on the telecommunications service that the company has purchased.
It might be that the company has a single number (ABC) DEF-XXXX, and usually some number of those Xs, usually three or four, is the extension. Actually in my current office, the last five digits constitutes the extension.
therwise, all 10 digits might be the shared phone number and each desk gets an extra set of digits as an extension that has to be dialled separately after the operator (live or automated) answers. I’ve had extensions of 2-4 digits long.
I think ten-digit dialing is universal in the US now. Back in the day, if you wanted a number in the same exchange you could get it by dialing the only the last four digits. The last place I could do this was Millbridge, ME back on 1978.
It’s echoes of 140 years ago when most of the US phone system was set up by Bell. Other companies varied which is why you can find period ads with short numbers on them like BEntwood - 232
Your telephone was connected by a pair of wires* to a central office (the exchange) which could handle as many as 10,000 lines coming into it. When you picked up your receiver, a light would light on the operator’s board, she would plug into it and ask your number, then plug in that line if it was in the same exchange, or call an operator in the other exchange and the two of them would connect you.
When you hung up at the end of the call, another light would come on and the operator(s) would unplug everything. Long distance calls were a lot more complicated and I don’t wanna get into that right now. Suffice to say, there were reasons you could pick up a phone and dial another number in the same city yourself long, long before you could direct-dial a long distance call.
*Two pairs wires were commonly connected but only one pair was used per line. This is why adding a second number to your house was no big deal but a third line involved somebody climbing a pole outside your house, finding an unused line to the exchange, and putting in a second line from the pole to your house.
Not quite, but it’s close. I’m in the near west suburbs of Chicago (708 area code), and we can still dial within our area code with just seven digits. But, AT&T announced, a week or two ago, that they would be phasing that out later that year, and implementing ten-digit dialing for all numbers in 708 later this year.
I’d say the one exception still in effect in the USA is at call centers with an 800 number. Often, the employees answering the phones are too far down the Totem pole to warrant their own phone number, ideally the problem is resolved on the first phone call with the first person you reach. But, there are really messy situations and you may need to work with the same person over multiple calls. That person would give you an extension that you’d have to specifically ask for when you get through on the 800 number
The general term for code systems with the property required to work like German telephone numbers is prefix codes and, as you may imagine, they come up in other contexts, such as machine code.
I worked at a place where the internal PBX was so obsolete that the only place to find parts was eBay, back in the day. Nobody supported it hardware-wise any longer, nobody made parts. When the head office decided the company would go VoIP, this location was second on the list. First on the list, was a location where all the “extensions” were separate phones installed and maintained by Bell, and each extension was actually one of a block of public numbers. Why them? Each line was costing the company $35 a month. Whereas, the obsolete PBX extensions were free.
So you’re saying that the first digit tells you how many total digits there are?
There are 7,919,900 potential numbers for an area code (it excludes things like 555-XXXX numbers). If an area code “runs out” of numbers, they can either:
- Shrink an existing area code, and assign a new 3 digit area code to new numbers in the previous area code. In this case you don’t get forced to reassign, only new numbers.
- Create overlay plans in the same geographic area, where all/most of the old area code is dried up.
For example, 415 was in the Bay Area. The eastern portion was split to 510, while the SF peninsula + Marin kept 415. Later the northern East Bay became 925. Finally 828 was created in the same area as 415 and 341 over 510. 925 is still by itself. Note that these areas don’t have almost 8 million people, businesses included.
How do you get that out of anything I posted?
By clicking on the link you provided and trying to understand it. Please, tell me what the other possibilities are, because if that’s not what a prefix code is, then I’m out of clues.
The basic idea of a prefix code is that codes are variable-length, but you always know when you’ve read a complete code without needing to explicitly mark the boundaries between codes. This works because no code is the first part, or prefix, of any other code. For example, if FA is a code, F can’t be a code, because you can’t tell F as a complete code from F as the first half of FA. However, if AB is a code, X can be a code and CD can be a code and CE can be a code, as well.
So apply this to phone numbers: If my phone number is 9876, no other phone number can start with 9876, because if any do, the phone system will only read the 9876 part at the very beginning and then connect the caller to me. However, if nobody has the phone number 9877 any number of phone numbers can begin with 9877, as long as, again, none of those phone numbers are the prefix of any other phone numbers.
The tree diagrams on the Wikipedia page might help you understand the concept even further.
Most phone systems will wait for more digits for a certain (short-ish) length of time, then assume you finished dialing and try to interpret the number. If you dial half a number and stop, you typically get an invalid number message or an error (fast busy) dial tone.
So as you dial, the system has to check at every step whether there is an end user with that number?
It’s more like the system is navigating a maze where each room has nine doors leading away from it, numbered zero through nine. Dialing a number is giving directions through the maze: In my example, it would go: “First, take door 9, then take door 8, then take door 7, then take door 6.” When you open door 6, you’re connected to me. You can’t keep dialing without making noise in my ear, because at that point you’ve left the “dial a phone number” maze. You can only keep dialing if none of the rooms you enter connect you to someone, see?
That’s the essence of the concept: If you dial a number which has another complete number as its first part, you get ejected early and can’t finish what you intended to dial.
But this is not the case for (German) phone numbers. I will only know if 9876 is a complete phone number if I dial it. If it is an incomplete phone number, I will hear silence (and a disconnect tone after a few seconds). If it is a complete number I will hear a beep-tone that signals if the called phone is ringing or if the line is occupied.
Yes, that is how it works. If you go back in time when rotary dial-phones where state-of-the-art, this system is easy to imagine. You pick up the phone and dial the first number, e.g. a 5. The phone then produces 5 impulses and sends them to the relay station of the phone company. Here, a first relay listens and counts the impulses and connects your call to the next relay that belongs to numbers staring with 5. This second relay then listens and counts the impulses of the next number you dial, e.g. a 0, which corresponds to 10 impulses (this is the reason why the 0 is placed after the 9 on a rotary dial phone). This goes on for the following numbers, e.g. 4 at the third place and 6 on the forth place. If 5046 is an existing number, your line is then connected through to the persons phone. If 5046 is incomplete, there are further relays connected to the number 6 outlet the particular relay that can will connect to further relays/numbers. In essence, this is how it still works, but I guess that nowadays there are no physical relays involved anymore.
Right, that’s what I said. The “you” there was the system consuming the code.
I still have a line right now where, if I simply dial a bunch of digits, it figures it out after a couple of seconds, but if I append a ‘#’ onto the end the call goes through immediately.
I think - the switching computer hears all the digits dialed - but when it starts interpreting the number, will stop when the number it interprets is “complete”. There are no numbers that are subsets of other valid numbers here. (or anywhere?)
If it’s local, nowadays its 10 digits, and the first 3 must be area codes valid for the local area.
If it’s long distance, dial 1 first. Then 10 digits. the first 3 must be a valid long distance area code.
If it’s beyond the continental area, it’s something like 011 first.
I am not aware of any systems where the switching computer will pass on the extension dialed after the 10 digits, so for further dialing you must wait until the connection to the internal PBX system is made. (I may be wrong)
For PBX where each number is part of a reserved block in the 10 digits, the switch system can forward the last few digits on the trunk line.
For example - the antique system I worked with - the numbers would be, let’s say each extension was 416-678-2xxx where 2xxx was your extension internally. If someone dialed any number in that range, the phone company switch forwarded the connection over one of a dozen trunk lines, and also forwarded the information of the last 3 digits dialed. This allowed the internal switch to connect the incoming trunk with the appropriate extension.
Internally, you dial the 4 digits starting with 2 (i.e. 2678) to get another extension. You had to dial 9 to get out - then wait for the outbound trunk to give a dial tone from the phone company switch to dial the 10-digit number using phone company rules. The idiots who dialed the full 10-digit number for another extension tied up two trunk lines, one inbound one outbound. For modems, “wait for a dial tone” was solved by prepending all outbound numbers with “9,” since the commas indicated pauses, and two pauses was long enough to get a phone company dial tone over the outbound trunk.
(There was a lot of extra programming they could do - what lines had outside access, etc. They could even set up a series of 3xxx extensions that could not accept direct dial in calls, since there was no corresponding outside number. There was auto-forwarding, and hunt - find the first line in a group list that was not busy; forward after hours; ring pickup group - dial a certain number and a group of phones would ring, so anyone at any of those numbers could pick it up. These are the sorts of tricks that make a PBX or VoIP equivalent more desirable than just simple collections of phones. )