Most common way would have been a phantom ext. By NIGHT MODE, do you mean a true NIGHT MODE where receptionist puts system in NIGHT when she leaves? If so, go to SYSTEM/DEVICES AND FEATURE CODES/CO TRUNK GROUPS (assuming you use conventional trunks). Pick the group whose name looks most likely to be your main trunks. Should see DAY and NIGHT destinations. If they say CALL ROUTING TABLE instead you will see tables as destination, open the night one and look at destination shown for your main number. Through one of these you should be able to see an extension, probably a phantom ext.
If you use a schedule for day/night, find the STAR application for the schedule: VOICE PROCESSOR/DEVICES/APPLICATIONS, STARs will be near the bottom. You'll find a DEFAULT application near the bottom. Click on it to get to the application listed, inspect DIGIT TRANSLATION, you should see an ext listed as TIMEOUT, again probably a phantom.
If you play some sort of message before going to phantom you'll need to inspect the application to see where the menu choice that goes to phantom is (if message says "press 1 for on-call tech", you'd look at digit translation for 1).
Either way, you now probably know the PHANTOM that is forwarded outside. You can change forwarding from a phone by pushing lots of buttons, but here's an easy way:
Top of the screen press VIEW, then ONLINE MONITOR. Dire warnings appear, ignore them, you're a skilled technician. Now open the phantom ext (SYSTEM/DEVICES/PHANTOMS/<ext in question>). Open FORWARD INFORMATION. You should see lots of stuff. Don't change anything except the FORWARD OUTSIDE NUMBER. That should do it. You can now test by just dialing the phantom from your phone.
This is probably the way things were done, but if you find something else let me know...there are several less-used alternatives and if you tell me what you find I should know what's happening.
