ETF NIGHTMARE!!!

So, customer service waived my ETF. Assuming I returned the phone. In addition, my complaint with the FCC and BBB yeilded a letter waiving the ETF. So I mailed the phone, using their UPS return pack. Fortunately for me I kept the tracking number. After a month, I got a bill for $300. I called in and the Sprint rep immediately said that they had no record of receiving my phone. SHOCKER!!. I panicked, as I wasn't sure if I still had the tracking number. Luckily, I found it. With that number, the rep immediately confirmed that they did have it. What happened, were they hoping they could bluff their way into $300 bucks? If I didn't find that number, they were ready to charge me. So, the rep informs me that the fee will be taken off. 3 weeks later, I get a notice from a collections agency. More to follow.