If you have converted from a 3-digit to a 4-digit numbering system and you still want people to dial the old way (3 digits): 

• Login to the system administration
• Navigate to 'Settings: Servers: Edit'
• Set 'Alias prefix'='1'
• Set 'Alias length'='4'
• Click on 'Save'

Now every time '234' is dialed, '1' will be added and the system will make a call to extension '1234'