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Understanding Digit Transform Rules and Digit Manipulation
This topic covers digit transform rules that you use to manipulate called numbers before they are routed out telephony trunks.
Many techniques exist for performing digit manipulation using digit transform rules. You can add digits, strip them, change them, and expand a telephone number.
You specify a digit transform rule as a regular expression. You assign a digit transform rule to a trunk group within the dial plan. You can use a single digit transform rule for multiple trunk groups in the same or various other route patterns.
When a called number matches the digit pattern of a route pattern and a digit transform rule is configured for a trunk group of one of its route patterns, the rule is applied to the digits of the called number to transform it before it is routed out any trunk belonging to the trunk group.
For example, it is common for organizations to require employees to dial an access code, such as the digit 9, to make outside calls. Using digit manipulation, you can remove the 9 from the called number before sending it to the PSTN so that the PSTN does not reject the call. It’s helpful to name rules to represent what they do. You might call this rule stripleading9. You might call a rule that inserts a prefix for a branch add-branch-prefix.
Route patterns specify different digit patterns to allow for different types of calls. If you use the same trunk group for multiple route patterns in a dial plan, you might want to manipulate the called numbers for one instance of the trunk group but not for another instance of it. You can do this because you assign digit transform rules to trunk groups within the context of a route pattern, not when you define the trunk group.
