From the customer view in https://admin.webex.com, go to Services > Calling > Service Settings, and then scroll to Internal Dialing.

Configure the following optional dialing preferences, as needed:

• Location Routing Prefix Length—We recommend this setting if you have multiple locations. You can enter a length of 2-7 digits. If you have multiple locations with the same extension, users must dial a prefix when calling between locations. For example, if you have multiple stores, all with the extension 1000, you can configure a routing prefix for each store. If one store has a prefix of 888, you'd dial 8881000 to reach that store.
• Steering Digit in Routing Prefix—You can set a value here regardless of whether you use location routing prefixes.
• Internal Extension Length—You can enter 2-6 digits and the default is 2.

  After you increase your extension length, existing speed dials to internal extensions are not automatically updated.

Specify internal dialing for specific locations. Go to Services > Call > Locations, select a location, scroll to Dialing, and then change internal and external dialing as needed:

• Internal Dialing—Specify the routing prefix that users at other locations need to dial in order to contact someone at this location. The routing prefix of each location must be unique. We recommend that the prefix length matches the length set at the organization level but it must be between 2-7 digits long.
• External Dialing—Optionally, you can choose an outbound dial digit that users must dial to reach an outside line. The default is None and you can leave it if you don't require this dialing habit. If you do decide to use this feature, we recommend that you use a different number from your organization's steering digit.

  Users can include the outbound dial digit when making external calls to mimic how they dialed on legacy systems. However, all users can still make external calls without the outbound dial digit.