Product: Debt collection

This complaint is because of a terrible mistake by XXXX which resulted to submitting a mistaken bill to a debt collecting company and this company ( XXXX XXXX ) has put a collection on my credit record over XXXX $ and dropped my credit score more than XXXX points from XXXX to below XXXX. Now, I am applying for mortgage loan to buy a home and due to this incident, I got rejected few times and now got approved with very high interest rate and down payment. I am a professional and responsible person. I have PhD of XXXX XXXX and working for XXXX XXXX XXXX and have never had single dollar overdue bill in my entire life. My credit score has always been above XXXX since the 1st time I got credit card. 

On XX/XX/XXXX, I sent a service cancellation request to XXXX to cancel my home internet service on XX/XX/XXXX. The service got canceled on time and I dropped off the modem to XXXX store in XXXX California on XX/XX/XXXX. 

My credit card was charged XXXX $ on XX/XX/XXXX for XX/XX/XXXX, until XX/XX/XXXX, although the service was canceled 12 days earlier on XX/XX/XXXX. I thought this is their policy to charge me for full month. 
Seems XXXX determined that I had an over payment and they sent me a bill showing $ - XXXX on XX/XX/XXXX. I also received a call from XXXX, and they asked for my new address to send me back the XXXX $ check. 

I never received the XXXX $ check. Because it was not considerable amount and because I just moved to a new home, I was very busy and had no time to follow up. 
On XX/XX/XXXX, my credit card was charged again by XXXX. This time XXXX XXXX I called the customer service and they apologized and said it may take few weeks or even months until the system gets updated, so you better to dispute the last payment. I asked about the XXXX $ that XXXX owed me, and she said it's related to the billing department, and they are different department than us and sooner or later they will figure it out and send you the money. 

Few months later, I started to get many calls from debt collector company. At first, I thought it's a scammer calls because they asked me to verify my name and home address, without knowing who they are, so I ignored them. Once I mistakenly answered the phone without knowing the number and the debt collector said it's about your XXXX $ unpaid internet bill from XXXX. I still didnt believe it's a legit call because I have never believed XXXX XXXX send a mistaken XXXX $ bill to a debt collecting company. I tried my best to explain to him that, they owe me money and it's a mistake, but the guy was like not hearing me and keep continue saying " So sir, you don't want to pay your bills? '' several times. 

I contacted XXXX customer service again and explained the situation. I talked to XXXX XXXX from Payment services and support, XXXX XXXX, located in XXXX UT. She listened to me and told me we will look into it, and she emailed me all the bills that I already had without any additional explanations. I told her it's very difficult to explain everything again to someone over the phone again and again and she said I will call you next week if we cant solve the issue. She never called and I thought, thankfully they figured it out. 

I had no calls until late XX/XX/XXXX and didn't answer the unknown numbers. Sometimes in late XXXX I got another call from the debt collector again and mistakenly picked up the phone. I told the lady all the story. I told her, seems XXXX has realized the issue and it's fixed, but again, she was like not hearing what I am saying. 

Last weeks I applied for a mortgage loan to purchase a home. I was told the debt collector ( WayPoint Resources ) has put a collection on my credit record over 52 . Which resulted to more than 100 credit score drop. I got rejected by few lenders and finally I got approved only if I pay very high down payment and interest rate. 
I would like to request for deletion of the collection tradeline off my credit record, so I can buy my dream home without having these troubles. I have attached all the bills, screenshots and emails that support my statement. 
Thanks,