3.7 Implementing PSTN Access

This topic describes how to implement PSTN access in a multisite environment.

Perform this digit manipulation at Cisco Unified Communications Manager (in the case of MGCP gateways) or at the H.323 gateway:

  • Outgoing calls (to PSTN):
    1. Calling number transformation
      • If no DID range, transform all directory numbers to a (single) PSTN number.
      • If DID range, add prefix.
    2. Called number transformation
      • Strip access code.
  • Incoming calls (from PSTN):
    1. Calling number transformation
      • Transform to complete PSTN number and add access code.
      • Consider TON.
    2. Called number transformation
      • If DID range, strip PSTN prefix to directory number.
      • If no DID, route call to attendant or IVR application.

When you implement PSTN access, you must perform the following digit manipulation before sending the call to the PSTN. You must perform the digit manipulation in Cisco Unified Communications Manager when you use anMGCP gateway, and you can do it either in Cisco Unified Communications Manager or at an H.323 gateway.

  • Outgoing calls to the PSTN:
    1. Calling number transformation: If no DID range is used at the PSTN, transform all directory numbers to the same, single PSTN number. If DID is used, extend the directory numbers to a complete PSTN number.
    2. Called number transformation: Strip the access code.
  • Incoming calls from the PSTN:
    • Calling number transformation: Transform the calling number to a complete number (considering the ISDN TON), and add the access code.
    • Called number transformation: If DID is used, strip off the office code, area code, and country code (if present) to get to the directory number. If DID is not used, route the call to the attendant or to the IVRapplication.

PSTN Access Example

The figure illustrates an example of digit manipulation that is performed for incoming and outgoing PSTN calls.

As shown in the figure, internal numbers must be represented as valid PSTN numbers, and PSTN numbers should be shown with the access code 9 internally.


Adding the access code (and changing 10-digit PSTN numbers to 11-digit PSTN numbers, including the long-distance “1” digit) to the calling number of incoming calls is not required. Adding it, however, allows users to call back the number from call lists (such as received calls or missed calls) without having to edit the number by adding the required access code.


This subtopic describes how to manage PSTN numbers based on their TON.

ISDN TON provides information about number format:

  • Subscriber
    1. 7-digit subscriber number
      • 3-digit exchange code
      • 4-digit station code
  • National
    1. 10-digit number
      • 3-digit area code
      • 7-digit subscriber number
  • International
    1. Variable length (11 digits for U.S. numbers)
      • Country code (1 digit for U.S. country code 1)
      • Area code (3 digits for U.S. area code)
      • Subscriber number (7 digits for U.S. subscriber number)

Number must be transformed differently, depending on the TON.

The TON is used to specify the format in which a number (such as calling number or called number) is represented. To have a unique, standardized way to represent PSTN numbers in Cisco Unified Communications Manager, the numbers must be transformed based on the TON.

For example, if the calling number of an incoming PSTN call is received with a TON subscriber, the PSTN access code can be prefixed so that the user can place a return call without editing the number. If the calling number is in national format, then the PSTN access code and the national access code are prefixed. If a calling number is received with an international TON, the PSTN access code and the international access code are prefixed.

In countries with fixed-length numbering plans, transforming the numbers is not required, because users can identify the type of calling number based on the length. In this case, users can manually prefix the necessary access codes. In countries with variable-length numbering plans, however, it can be impossible to identify whether the call was received from the local area code, from another area code of the same country, or from another country by just looking at the number itself. In such cases, the calling numbers of incoming PSTN calls must be transformed based on the TON.

ISDN TON Example: Calling Number Transformation of Incoming Call

The figure shows an example of performing TON-based digit manipulation on the calling number of the incoming call.

Site TON Calling Number Required Calling Number Transformation
1 Subscriber 5551111 9.5551111
2 National 7145552222 9.1714555 2222
3 International 49404132673333 9.01149404132673333

In the example, the main site gateway receives three separate calls, and callbacks should be possible without requiring the user to edit the number. The first call is received from the local area with a subscriber TON and a seven-digit number. This number needs only to be prefixed with access code 9. The second call, which is received with national TON and 10 digits, is modified by the addition of access code 9 and the long distance 1, all of which are required for placing calls back to the source of the call. The third call is received from another country (Germany, in this case) with an international TON. For this call, the access codes 9 and 011 must be added to the received number, which begins with the country code.