sundance-communications.com/forum Forums General General Telephone & Systems Information 4 digit dialing question

This might be a rudimentary question, but I will ask it anyway. Can someone explain how 4 digit dialing works? For example, when I call a co-worker using the last 4 digits of their phone number, does the call stay within the "office" by way of a local PBX or Base PBX? Basically, does there have to be some sort of PBX on site that handles it?

Well, let me try to help and I'll let the others correct me if I loose my way.

Yes, as far as I know you have to have a PBX on site. You would have a hardware in your "PBX Room" generally a carrier cabinet which holds all types of cards that would run your phones, voicemail, trunks, etc. One of those cards would be a D-I-D Card (Direct Inward Dial). This card would allow calls to come directly into a telephone and would allow you to use the last 4-digits per internal dialing.

You would need to contact your dialtone provider and order a bank of D-I-D numbers. Hint: always order more then you anticipate you will need. Then you add those numbers to your switch via your dial plan.

Of course, how you program these numbers and administer the extension/DID would depend on your PBX brand but this should give you a place to start.

The only exception would be if you were using Centrex.

If I'm not mistaken, say you have 555-0001 through 555-0100

Say the next office over it 555-0002, you pick up your phone and dial 0002 its just their local extention, but if an outside caller dials 555-0002, it comes into the PBX and that phone is assigned to that number so it rings extention 0002

Please correct me if I'm wrong.

If you're talking Centrex, there can be large networks. Some state agencies have all their office connected to a Centrex system. So yes they can dial a 4 digit number and get someone within the system in another location. But as you stated the outside world dials 7 or 10 digits and gets to the correct office via the Centrex. DID would be switch (customer switch) specific, so again with-in the switch 4 or whatever digits go to that extension in the PBX. Outside customer gets routed via the DID number, not necessarily the same number as the 4 digit extension, but it can be. Hope that makes a little sense.

I deal with Centrex (Plexar) every day....and Bill is exactly correct in his explanation. I havn't worked much with a PBX, so my knowledge with them is limited. But I imagine the 4 digit dialing for a PBX isn't that much different then what I'm used to working with allready.....just a smaller version (I may be over-simplifying?)

Bill's mention of government Centrex accounts is a good depiction of how 4-digit dialing is used. Centrex is especially popular in smaller towns where one central office serves the whole town. In larger towns or cities, it can be 5-digit dialing, but rarely more than that. It's exactly as described; dialing to another (internal) extension or department only requires the last four digits of the 7/10 digit published number. When calling from outside the Centrex, the full 7/10 digit number must be dialed. For extensions within the Centrex account to dial outward calls, they must be prefaced with a "9" or similar outside line access code.

This is exactly the way a PBX works. The difference is that a PBX (switch) is usually an on-premise piece of hardware that is paid for or leased by the end-user. Centrex a service that equates to a "slice" of the local telco's switch that can't be bought; it's just rented on a monthly basis. This means that every extension has a monthly expense, comparable to the cost of a dedicated phone line per-phone, but at a reduced rate.

Maybe someone else might want to chime in and un-confuse my explanation?

OK Ed, Centrex is like having the PBX at the telco instead of at the customer's building. Dial a 4 digit extension for anyone on your Centrex account, dial 9 to get out of Centrex to call anyone else.

Clear as mud?

If I am not mistaken, didn't the first PBX's come about as a means to basically bring CO (exchange) features (like centrex, 4 digit dialing, etc) into a private setting, hence Private Branch Exchange? I also think that this is the reason that it is still normal to dial 9 to get an outside line on many, many phone systems. They were set up that way originally to emulate the way centrex functioned.
In most cases, I think that 4 digit dialing appears to function about the same to most end-users whether they are on Centrex or DID's run into a PBX, although the hardware and the manner in which they each actually function may be completely different, as a lot of PBXs simply emulate centrex to one degree or another at the basic end-user level.

So... to answer the original question. It could be either an on-site PBX or centrex.

With Centrex all of the served telephone numbers are translated into the dialing plan of the particular Centrex. They are usually built as a 4-digit number using the last 4 digits of the individual telephone number. If it is a large Centrex, 5 or more digits may be required. If you call another line within the Centrex the call routes to the local telephone company switching office in the same manner as any other call. It is routed back to the called party from the local telephone company switching office. The fact that the Centrex lines all have the same unique Centrex identification number assigned and that the telephone numbers are built in to the Centrex dialing plan allows the local telephone company switching office to process the call using the 4 digits without the NNX. No Centrex call truly stays in the “office”. The exception would be if the Centrex were simply providing dial tone to some type of on premise PBX. This used to happen because the rate for Centrex was actually cheaper than a flat rate business line or a DID trunk.