Unfortunately I think your options are limited. You can sue them on your own in Small Claims court or hire an attorney to handle the matter.
I provide a free 15 minute telephone consult for security deposit claims and eviction defense. No attorney-client relationship is created by answering questions in this public forum. If you wish to create an attorney-client relationship, you must contact me directly and sign a representation agreement. Answers are provided based on general ideas and an answer specific to your situation would require a review of all documents.
Here's your problem. You signed a contract that likely makes you liable for their "reasonable" (or unreasonable) charge. You can go the lawyer route but not the small claims route yet because you haven't been sued (small claims courts cannot issue declaratory judgments).
Start the letter writing campaign to the company. Send a complaint to the BBB and the State of Florida Consumers Affairs people (I believe the Ag Dept. check the web). Make phone calls and write down the names of everyone you speak to and their response (include date and time). Write back to the collection agency and dispute the claim (again, the web is a wonderful resource for form letters and process). In short, any lawyer will cost you $500 at least to crank up the engines of war.
Try to play the consumer card. Don't give up and become a thorn in their side.
This is not legal advice. Think of this as what you would recieve from a wise uncle. And remember, you have never believed everything your uncle told you either. With free advice you generally get what you pay for.
Try to speak with a manager and see if you can get this amount waived or drastically reduced as much as possible. If that does not work, follow the advice of the other lawyers. Good luck!
The biggest issue in your facts is the documentation that in November you were told no ETF fees as long as you returned the phone. You may have fallen into the old" I called them lots of times but couldnt get an RA number and envelope" issue where there is no documentation for your calls. You should immediately get a call listing from your phone service showing the LD calls or if on a cell the existence of the calls made to prove you called. Otherwise you get stuck with the argument that they keep billing until you return the phone and they make it look like delay was YOUR fault. Also check to see what current bill claim is now as the last transaction may have crossed over and a later posting may have given you credi for the phone return etc.
If its CLEAR they are charging you for services that should not be billed, you should look at the Fla. Stat 559 FCCPA as it may afford you a remedy for any wrongful collections effort.