Complaint Review: Centurylink

Centurylink is billing me for 2 modems at $100 each and threatening to turn off my service if not paid.  As the only modem I have in my possession is the one hooked up to the internet, I asked for make, model or serial number of the modems I am being charged for and they cannot provide any information on the modems.  After 3 hours on the phone with all but one very polite customer service representative from 3 different departments, nothing could be found on the modems I supposedly have in my possession and am being billed for.  I do not have any other modems and cannot even provide a reciept for a defective modem I returned last month due to the fact they cannot cross reference the serial number with either of the modems I am currently being billed for ( because they do not exist).  My wife is in college taking online classes and is in the middle of finals, if they turn off the internet service, she will most likely fail the classes....right now she has an A in both of them.  Sounds a bit like extortion to me! I have filed a complaint with the FCC, I have never done that before, lets see if they can help!!??  

Any advise?  Thanks for reading!