Under the OGM properties window in the Data Entry options you can enable the 'Limit acceptable values' option which allows you to specify what extensions are allowed.
You can either choose to provide a comma delimited list of allowed extensions (e.g. '101,102,103,104') or choose to limit the range (e.g. between 101 and 104).
If the caller inputs an invalid number, the system will tell them their entry was invalid and ask them to enter the extension number again.
You can change the 'invalid number' audio under IVM's settings -> Prompts tab.
