Directory numbers for the Cisco IP phones are expected to be entered in extension number format. The extension number should be greater or equal to the extension length. Otherwise, the extension number cannot be converted to a qualified E.164 number. The dialplan-pattern command creates a global prefix that can be used to expand the abbreviated extension numbers to fully qualified E.164 numbers. The dial-plan pattern is also required to register the Cisco IP phone lines with a gatekeeper. The dialplan-pattern command can resolve an incoming call with a full E.164 number to a Cisco IP phone extension number.
The extension-length keyword enables the system to convert a full E.164 telephone number back to an extension number for the purposes of caller-ID display, received, and missed call lists. For example, a company uses extension number range 5000 to 5099 across several sites, with only the extensions 5000 to 5009 present on the local router. An incoming call from 5044 arrives from the company's internal VoIP H.323 network and this call includes the calling number as 4085555044 in its full E.164 format.
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