The Significant Digits feature instructs CUCM to analyze the configured number of digits (from right to left) 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 party number for routing incoming gateway or trunk calls. The Significant Digits feature is the easiest approach to convert incoming PSTN called numbers to an internal extension, but the setting affects all calls received from the gateway. The Significant Digits setting also assumes that the internal dial plan is using the last five digits (or other number specified) of the DID block as the internal extension (directory number). The Significant Digits setting also cannot accommodate variable-length extension numbers on the internal network. Variable-length internal extensions could also lead to a variety of overlapping dial plan challenges.
The PSTN gateway illustrated in Figure 11-14 is using the Significant Digits setting in CUCM to instruct CUCM to only analyze the last four digits of the incoming call with a called party number of (408) 555-1010 received from the gateway. The significant digits configuration is available in the gateway or trunk CUCM Administration configuration pages under the Incoming Calls section (toward the bottom of the gateway/trunk web page).