I have a few questions for you about the time.
You have said the between 6:00 am and 6:00pm you are getting the "PRE RING" on your sets. What happens after 6:00 pm and before 6:00am? does everything work correctly?

Does this voltage issue continue past 6:00 pm?
Do you know if these line are comming in over AFC? (or can you find this out)
Let me know if they are running that terminal with fiber & AFC.