Changing Mitel 5000 from 4 digit extension to 3 digits

[mitel15]

By default, the phone system is set to 4 digit extension. How do I change it to 3 digit extension?

Thanks

 

[619Tech]

Enable On-Line Monitor, then go to 5000\System\Numbering Plan\Device Baseline Extensions: Change Phone and Single Line Extension values from 1000 to 100.