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How the telephone exchange system communicates to your PABX and some important notes about call costing

@Here we introduce the terms PSTN, ISDN and PABX System. The acronyms look and sound impressive, but describe concepts that are not difficult to understand at a general level. Check the glossary if you are not familiar with these terms.

Note that NETWIZ provides a maintenance service that includes updates to the cost tables.

The MonTel software can receive data from your company's telephone system in a number of ways, TAPI, Files or through a serial port (RS-232). In all cases these are implemented as a one-way communication, where the PABX sends data to the MonTel system, and not the other way round. This means that the MonTel software can only report what the PABX tells it.

In turn the PABX can only report what the main telephone exchange tells it.

The main telephone exchange can communicate to the system through ISDN lines, and not PSTN. When you have PSTN lines, the main telephone exchange will only place or connects calls. It does not signal to your telephone equipment (PABX) that the call has been answered, or if the called line is engaged, or even disconnected.

Some telephone authorities have special devices that can be fitted at the main telephone exchange that will send signalling information to your PABX, called metre pulses. There are two types that can be used. However, they are not always available and depend on the compatibility of both the main telephone exchange and your equipment, indeed some PABX's may not be able to read such signals. Not only that but you are likely to be charged for the service. Therefore, metre pulses are not really suitable for this purpose.

So how does your PABX know if a call was answered ?

The simple answer is it does not know if your call was answered at all. It has to guess. Most systems do this by allowing a grace period of about half a minute (24 seconds is typical) for each outbound call before reporting it as a call. This means that if you wait for a call to be answered for more than 24 seconds it will be recorded as an outbound call (and costed as such). If, on the other hand, you are quick enough to complete an entire call (from time of last digit dialled to hangup) in less than the grace period the call will not be recorded at all. Note that most systems set the grace period for international calls to a few seconds.

Does this mean that you are billed for these calls that did not connect ?

The answer is no since the main telephone exchange knows whether a call was connected, it just does not have a way of communicating this to your (PABX) system.

You can get accurate billing information (to the second in many places) via the AOC on some ISDN lines, but not on Telstra OnRamp.

In Australia, ISDN and Telstra OnRamp local calls are timed, so the duration of the call is vital to determine the cost of the call. The AOC on the older ISDN system is not necessarily the same figure that will appear on your bill - so it is still worth checking even that information.

MonTel provide a different set of tables for PSTN and ISDN. Since the PABX usually reports the type of trunk line it used, the MonTel system can know if it should guess the cost of the call, or report accurately the figure presented by the PABX. Since the PABX has no information (usually) to report about PSTN lines, MonTel attempts to guess the figure based on the estimated time of the call, the destination number and the time of day.

The reported cost result from the look up table can only be a guess since it cannot know the exact time of the call, or even if the call was answered.

This is largely due to not having any information about the call being connected (or not). Most PABX systems subtract 24 seconds or so from the duration (which is otherwise the time from the dialling of the last digit of the number).

Of course, for ISDN calls it simply reports the information received from the main exchange. With spot discounts and special offers, plus constant price reductions and new plans with competition between telecommunications carriers the costs in the look up tables may not be up to date either.

It is important to note that the above distinctions are true for any cost recovery or reporting system.

All cost recovery systems suffer from the same problem of not being able to accurately charge calls.

Many will not tell you about it. Since MonTel is "live" or interactively reporting the information to the user when it happens, rather than later in a ream of paper, it is the only system (currently) that you are likely to easily notice these defects in PSTN lines. The costing provided by the MonTel system is a guide ONLY and should NEVER be taken to represent an amount that might be charged on a phone bill.

Without special equipment and an instantly up to date cost table, accurate cost recording cannot be made by any system. Even the data channel from the ISDN network cannot tell you everything.

Here is a summary of the reasons for call costs only ever being an estimate:-

  • The exchange does not tell your phone system if a call was connected (without leasing special equipment). Therefore, the duration of the call cannot be calculated.
  • Mobile phones, satellite phones, radio phones and others are charged differently depending on where they are at the time of the call. Your telephone system cannot know where they are.
  • Your phone system cannot know of spot specials, one off discounts or special rates that may apply from time to time, or be in place at your business.

It is important to understand that this is a generic feature/problem of phone systems and not a software fault in MonTel.