Wow, that is rough. I took a quick look through their terms and conditions and you may very well be screwed. First thing that caught my eye:
"When You activate your Straight Talk service, you will be provided with a telephone number or you may, in some circumstances, port-in a number from another carrier. Please note that You have no ownership rights to any telephone number, IP address or any other identifier associated with your wireless service and You acknowledge and agree that we may change any such number, IP address or other identifier associated with your Straight Talk service at any time without prior notice to You."
So they own your number now.
Refilling Your Account after Service End Date: If you do not buy and add Services prior to the Service End Date, your Straight Talk Service will be deactivated on the Service End Date, you will lose all unused Services (including minutes, text, Mobile Web Access and Service days) and you may lose your telephone number.
It sounds like even if you had waited 1 day after service end you ran the chance of losing your number.
When selling mobile phones about 7 years ago, we were always advised to tell business customers to stay away from prepaid plans for exactly this reason. There is a lot less protection for the customer when you are not on a contract, and the way prepaid companies are set up enables them to run free on a lot of these issues.
I'd recommend you keep calling until you get to their highest level of support (I think I heard they have a Miami office) and start threatening to file reports with the proper agencies. Unfortunately I read somewhere else that Straight Talk/TracPhone/Net10 already have a huge backlog of unresolved complaints, so it may not get you anywhere either, especially as the terms and conditions do spell out what their rights are.
Good luck. If you get your number back you may want to think about porting it to Google Voice immediately.