Significant Digits

The Significant Digits feature instructs CUCM to analyze the configured number of least-significant digits of the called number for incoming calls received by a gateway or trunk. Setting the Significant Digits to 5 on a PSTN gateway instructs CUCM to ignore all but the last five digits of the called number for routing incoming PSTN calls. This is the easiest approach to convert incoming PSTN called numbers to an internal extension, but it affects all calls received from that gateway. If there are variable-length extension numbers on the internal network, using significant digits is not the recommended approach for changing PSTN received digits to an internal dial plan.

In Figure 12-14, the PSTN gateway receives an incoming call with the destination number of 408 555-1010 through a gateway. If Significant Digits = 4 is configured under the gateway configuration in CUCM, CUCM strips off all digits except the last four (1010), looks up this number (1010) in its call-routing table, and forwards the call to the IP phone configured with that DN.

Figure 12-14 Significant Digits Example

Figure 12-14 Significant Digits Example

NOTE In contrast to using translation patterns to map E.164 numbers to internal DNs on incoming PSTN calls, this solution performs only one call-routing table lookup. The Significant Digits feature is a more processor-friendly alternative than translation patterns, but this approach will not allow the same flexibility as translation patterns.

You can configure significant digits in the gateway or trunk configurations under the Incoming Calls section. This parameter affects all calls received by the gateway or trunk.

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