If your system currently goes in and out of night service automatically it must have a STAR programmed.
Log into your system and go to Voice Processor / Devices / Applications
Within the main window you should see an entry numbered 25xx (default numbering) with a type = STAR, double click this and you should see 20 options then a Default application, the default application is where calls go to in night service (probably) one of the twenty options should have a description if it's been programmed properly.
Double click this entry and within this you should see a destination and what times calls will be directed to this destination.
Change the times within here!