Sprint I cancelled my cell phone service with sprint and transferred my service and phone number to T-Mobile but sprint kept charging los angeies ca California

I did not have a contract with Sprint, I was on a pre-paid plan. I cancelled my cell phone service with sprint and transferred my service and phone number to T-Mobile. However, Sprint kept charging me for months after. I signed up for pre-paid service with sprint on 6/27/16. On that day I purchased the cell phone for 303.32 and paid for the first month of service which was 113.72. Over the course of a year and a half my cell phone bill with sprint continued to rise for unknown reasons. I decided to switch my service to T-Mobile. My last payment to sprint was on 10/12/17 for 117.42, I pre-paid for October. T-Mobile is $50. a month Vs sprint which was 117.42.

On 11/6/17, I canceled my service with Sprint and started with T-Mobile. My phone number was transferred to T-Mobile. Sprint sent me a re-turn package because they wanted the phone back, which was an LG 3. Even though I paid for the phone and I have the receipt, I sent the phone back. (I didn’t need it) However, Sprint kept charging me for service. I received a bill demanding that I pay $117.54 for October. $117.42 for November. $85.94 for December, it kept going.  I told the sprint bill collectors I cancelled my service, and they said it was for "usage" but I did not use sprint service because I cancelled it. I also sent the phone back to sprint. SO HOW COULD I USE IT? On top of that they started charging me for late fees. Now my total charges with sprint is $327.00. I just started receiving calls from a collection agency. Sprint has reported me to the credit bureaus. I asked the collection agency to stop calling me, I even wrote a letter asking them to stop calling me. But they still are calling me.