Who/what generates the dial tone?

When I pick up my office phone, I get a dial tone. I hit “9” to get an outside line, then after a brief silence, get another dial tone.

Does the dial tone come down the line from the phone co.'s switching station, etc.? Is there a separate tone being generated inside my office bldg. for intra-office calls?

The first dial tone you get is from your office phone system, or PBX. It is used for dialing internal numbers. Once you dial 9 the pbx knows you want an outside line, but doesn’t know which line to use. As you start to dial the number it looks at its routing tables and sees which “trunks” to use. These “trunks” are groups of outside lines. As soon as the pbx knows which trunk to use it queries the central office to see if they are available and then passes the call over to the co (central office). That is the standard way of configuring your system. You can also set it up so that 9 accesses one specific trunk group and in that case once 9 is dialed the dial tone is coming from the central offie. There are other methods to access one specific line which gives you direct co dialtone.

Basically the dialtone is coming from your pbx until the pbx has enough info to pass the call to the central office. After that it is central office generated. Also this is somewhat dependent on what type of system you have also. The older systems are more likely to be set up with the 9 as a direct access to a specific trunk, rather than using route patterns.

I apologize for being long winded here, however, this is right up my alley.

PBX/Voicemail Technical Support Engineer

Can a PBX system work if the company (or whatever) is located at more than one site, eg. our hospital has three geographically distinct sites (all in the same area code). Could we all be part of one big happy PBX family?

Where I work, the geographically distant sites are connected by tie-lines, and you have to dial a 3-digit prefix plus the person’s extension to dial them at another site. So it is possible, but I don’t know the details of how it works.

Yes, it can. Two different ways we did it:
[li]My old company moved to Alexandria, but needed to keep a small office in Arlington. The VP wanted the receptionist able to take calls and route them to employees at both sites. We bought 2 PBXs, one for each site. The main site was a bigger model and had the company’s voice mail unit. The satellite office had a smaller model PBX. They were tied together with TIE lines, leased lines that would seemingly act as direct wires (but with some programming exceptions).[/li]
The main office’s extensions went from 3001 to 3199; the satellite office had 3200-3299. The main PBX was programmed to route 32## extensions through the tie lines, sending a parameter of 2## (the local extension number). The small PBX would then connect the incoming TIE line call to extension 32##. It worked in reverse for the small office calling the main office.

Each office had a set of outside lines so as not to choke each other’s capacity, nor the TIE lines.

[li]Before the above arrangement, we needed to open a cheaper cost center for a new contract. So we rented another suite in the same building, but at a cheaper price. We then had a bundle of wires connected to the patch panel for our main PBX and routed down through holes in the utility closets to our new suite. There, they were attached to another patch panel, which was attached to RJ14 jacks throughout the suite.[/li][/ul]

The agency I work for is currently distributed among about a dozen buildings, the distance from the northernmost (where I work) and the souternmost is about a mile. All our offices either have a 308- or 305-exchange; when 10-digit dialing was initiated here, we had a few problems dialing within our exchanges but they seem to have been fixed. Dunno if we all have one huge PBX or not, we probably have more than one judging from those snafus.

One way to achieve inter-office communication is to use “Centrex”.Instead of having a PBX located at your premises,the centrex is a small part of the main,central exchange located at the telephone company’s headquarters. This acts like a private PBX,with extension to extension dialling,call following,etc but it is tie-lined out to your own premises.If you have more than one building or several locations around town then it is quite easy to run tie-lines out to all these places from the Centrex exchange.

This is the most common method I see. It is called DCS or distributed communication system. Another option is to set up the tie lines using ISDN-PRI lines. The lines are administered in the CO as direct connect lines and only pass the required , 4, or 5 digits. Because they are ISDN you can still get the calling party info such as name and extension on your phones display, but there is minimal administration in the pbx. This has the advantage of not having to buy a license to use the dcs software.

Another option that is gaining ground on this is to have 2 pbx’s and connect them both to a lan and communbicate between them using VOIP technology, or Voice Over IP. This is an inexpensive option because you are not having to pay for any tie lines. the problem with this is that the technology is still fairly new and has some bugs, such as choppy speech if the network is overloaded. I personally think that as this technology grows and improves it will take over.

A third option is to use an ISDN-BRI line to the remote location and what is called an extender. The extender is a piece of equipment that connects to the isdn line and allows your phone at the remote site to act as if it was onsite with the pbx. This is usually only used if there are only a few people at the remote site. It requires a line in to the pbx for the extender to communicate with it and any calls made from the remote site using the extender are routed back through the main pbx.


Very interesting. Did you also know that you can tune your guitar to a dial tone? It’s an A.

The company I work for has the ability to dial some of our overseas locations by simply dialing a 3 digit extension. This is done through a satellite connection, as the land lines in some of our locations are terrible. It was expensive to set up, but it proves cheaper on a month to month basis.

Thanks for all the input and insights. Now I can speak with (a bit of) confidence the next time one of the operations people says “no way”.

(It is a huge pain to work as we do now, with three separate phone systems.)