"How do I find out what extension has mailbox 189? When someone leaves a general message, I do not see any message light come on at any of the extension. After a while, then I see the light come on at extension 102 with the "Message transferred from mailbox 189"."
I'm not sure what your asking. Mailbox 189 does not have an extension. There <probably> isn't an extension 189. To log into mailbox 189, you would have to dial the voicemail's extension (ususally intercom-165) then exit to the voicemail service ( # + 6 ). Then dial * + 189 to log into the other mailbox.
This is not going to help you change the forward time. You have to do that the way a technician would, either through a PC terminal, or through mailbox 999.
"Also, how do I setup so certain extension will not ring at night?"
At this point, I get the feeling you haven't programmed this system before. But if you can program it, and you have a "Night Mode" setup, you can add and delete extensions that ring under "DIL 1:1 N".