This document covers the basics of implementing the PSTN Access Dial plan in Cisco IOS Gateway and the Call flow between the Cisco Unified Communications Manager (CUCM) and the Public Switched Telephone Network (PSTN).This document is intended to help the beginners to understand the basic concepts of PSTN access and call flow.
Implementing PSTN Access in Cisco IOS Gateway
When you implement PSTN access in a multisite environment, you must perform digit manipulation, described in the following list, before the call is sent to the PSTN. Digit manipulation must be done in CUCM when you use an MGCP gateway. It can be performed either in CUCM or at the H.323 gateway when using an H.323 gateway.
1. Outgoing calls to the PSTN:
ANI or Calling Number Transformation: If no direct inward dialing (DID) range is used at the PSTN, transform all directory numbers to a single PSTN number in the ANI. If DID is used, extend the directory numbers to a full PSTN number.
DNIS or Called Number Transformation: Strip the access code.
2. Incoming calls from the PSTN:
ANI or Calling Number Transformation: Transform ANI into the full number (considering type of number [TON]), and add the access code so that users can easily redial the number.
DNIS or Called Number Transformation: If DID is used, strip 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 an attendant, such as a receptionist or an interactive voice response (IVR) application.
Figure shows an example of digit manipulation performed for both incoming and outgoing PSTN Calls.
PSTN Access Example: Incoming and Outgoing call flow between CUCM to PSTN
Figure shows an example of digit manipulation performed for both incoming and outgoing PSTN calls. Internal numbers have to be represented as valid PSTN numbers, and PSTN numbers should be shown with access code 9 internally. ANI is the number calling from and the DNIS is the number calling to.
Subscriber: Seven-digit subscriber number: three-digit exchange code, four-digit station code
National: Ten-digit number: three-digit area code, seven-digit subscriber number
International: Variable length (11 digits for U.S. numbers)
Country code: One digit for U.S. country code; one, two or three digits for all other countries
Area code: Three digits for U.S area code
Subscriber number: Seven digits for U.S subscriber number