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CUCM Prefix Digits

“Prefix Digits” is a dialing transformation for both inbound and outbound calls. It allows to prepend the called number or the calling number with a set of digits before sending the call to the next stage of call routing. This next stage can be the call routing component (if the call is inbound) or a gateway/trunk (if the call is outbound).

On one hand, Prefix Digits is a feature that can be applied to the digits received on inbound calls, for the following ports and gateways:
– ports: FXS, FXO, E&M, digital access PRI and BRI
– gateways: H323, H225 trunks
On the other hand, in Calling Party Transformations and Called Party Transformations, Prefix Digits is applied for outbound calls only. That’s why we see the field named  Prefix Digits (Outbound Calls) in CUCM. Since Calling and Called Party Transformations are applied in route patterns, translation patterns and Hunt Pilots, we can say that Prefix Digits in these constructs is applied for outbound calls only.

In Prefix Digits -or Prefix DN- The choice of the digits to prepend is any combination of the following alphanumeric characters: *,#,0-9,+.

Prefix Digits configuration in CUCM

In CUCM, Prefix DN feature can be configured on digital gateways and trunks.

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