An Auto Attendant must be setup to allow remote access, and it must be enabled in each extension's Enhanced Services. When the Remote Access option is selected, the caller will be asked for their extension to use for remote access.
In the options for Remote Access, the Require Extension PIN checkbox should be checked for security reasons. This will require a PIN to be entered after their extension before allowing remote access.
You can also add phone numbers to the Caller ID list which will allow calls from those numbers to be automatically directed to this extension. You can also optionally require the PIN or not from that Caller ID. This is useful for allowing a personal cell phone quick access to someone's personal extension.