Product: Debt collection

Amsher Collection Services the collection agency for XXXX is charging me {$270.00} in error. I have paid my XXXX cell phone bill in full. In fact my final bill from XXXX for XX/XX/XXXX XX/XX/XXXX states XXXX owes me {$44.00} which I havent received yet. I called in to XXXX in XX/XX/XXXX because I havent received my final reimbursement check, which then I was told by the XXXX rep that I owe {$270.00} charge for a mobile device/signal booster that was never returned. I then informed rep that not only did I return device back in XXXX a week after it was sent to me because it didnt work properly. I also inform XXXX rep that back in XXXX I called XXXX customer service because I checked my statement and I was still getting charged for the mobile device/signal booster every month even though I returned it in XXXX. 
The XXXX rep in XXXX did see that I did attempt to return device and that the device possibly was lost in transit. That rep immediately credited my acct I believe around {$72.00} for the monthly fees I was being charged and removed that mobile device/signal booster from my acct completely and informed me going forward I was no longer was being charged for said device. My monthly statements shows the device being removed. I did not have any issues with XXXX until I requested to close my XXXX acct in XX/XX/XXXX. Now after I close my acct this random {$270.00} charge appears in error. I informed the rep back in XX/XX/XXXX of the error, and she informed me I was correct and she would fix my acct and there wasnt anything further I have to do. Then I believe in XXXX I receive a call from Amsher Collections that my XXXX acct was now transferred to them and in collections. I was livid, I explained to Amsher collections that this is an error on XXXX behalf and please dispute charges. I received my XXXX statement in the mail from Amsher Collections which shows XXXX FINAL BILL Im owed {$44.00} credit then I also received {$270.00} bill from XX/XX/XXXX XX/XX/XXXX says I owe {$270.00} with no details of owed 2 pages were missing. I have since called XXXX over 20 times within XX/XX/XXXX until XX/XX/XXXX with multiple reps saying they would correct my acct and never fix it. One XXXX rep informed me Ashmer Collections would have to fix it. I have also sent copies of my final bill to XXXX XXXX supervisor XXXX on XX/XX/XXXX and never received a call back or any correspondents. Within the past month XX/XX/XXXX I have spoken to over 5 separate reps who promised to call me back and never did. I started writing down names the week of XX/XX/XXXX I spoke with XXXX, XXXX, and XXXX. XXXX a supervisor who promised to call me back who never did, I had to call in to speak with a supervisor on XX/XX/XXXX and XXXX XXXX supervisor finally called me back on XX/XX/XXXX to ask for additional proof which I informed her to please review the call back in XXXX which the rep clearly refunded me back money for said device and removed device completely from my acct. I also asked her to review my statements which shows the device being removed from my acct. I requested for all my XXXX statements from XXXX XXXX to show that XXXX removed device back in XXXX and I still havent received any of my requested statements. I now feel that XXXX AND Amsher Collections are intentionally avoiding my calls/hanging up and not assisting me. It shouldnt take 6 months to correct the error XXXX caused. My last call to XXXX customer service was on XX/XX/XXXX because XXXX a XXXX supervisor said she would call me back within 72 hours from XX/XX/XXXX and another week has passed without any follow up. I was on the phone with XXXX rep XXXX who was supposed to transfer me to her supervisor and the phone hung up after a 2 hour wait. No one at XXXX nor Ashmer collections has been helpful. This acct now being in collections has caused my credit score to drop over 70 points and is affecting me purchasing a property and my lending credit. All because of an error XXXX promised me they would correct every time I called in XX/XX/XXXX, XX/XX/XXXX, XX/XX/XXXX, and XX/XX/XXXX. Please hold these companies accountable for their bad practices. Thank you