1. Login to the system administration.
2. Navigate to Settings: Servers.
3. Click on the edit icon corresponding to the server.
4. Navigate to the "Numbering Defaults" group.
5. Enter a value into the "Number of digits" field.
6. Click on the "Save" button

NOTE: Before you can change number of digits on the system, you need to delete all the extensions, IVRs, Queues, Conferences, and Voicemails.