You may sometimes need to allow 1 or 2 digit extensions for subcribers/handlers. Typically 3 is the minimum for performance and network numbering conflict reasons. It is not a good idea to have fewer than 3 digits in your extensions but if a rare case comes up where it is necessary, you can adjust it in the Windows registry. You can also change the minimum to greater than 3 if you wish to make sure admins follow a 4 or 5 digit numbering plan for instance.
You can set the minimum extension length required for subcribers/handlers in the Windows registry.
HKLM\ SOFTWARE\ Active Voice\ Doh\1.00\
DWORD: Minimum Handler AVP_DTMF_ACCESS_ID length