Product: Debt collection

The XXXX have never send us the corrected final bill. We used theirs services for less than 2 weeks. We have returned theirs phones as well. They have 2 weeks no charge policy for new customers so we do not owe them anything but {$50.00} activation fee related to one of the phones we have activated. 
( We needed to cancel services with them because what we were told we will need to pay each month was a way different than what they have send to us on paper statement. ) We have returned both phones but they send us back one of them. 
We send the phone back as soon as we could. 
We do have a receipt from XXXX  from XX/XX/2019. And we do have a confirmation that the phone has been delivered on XX/XX/2019 at XXXX XXXX. 
However the XXXX didnt stop sending us bills for services never used and the phone that has been returned. 
We called them to discuss the charges in XXXX/XXXX/XXXX. Before we received the letter from a collection agency XXXX employee told us that actually all we have to pay to XXXX is {$50.00} and he can see that XXXX received the phone back from us. 
We were ready to pay over the phone but we were told to wait for final bill. Paying over the phone was not an option. 
We needed to wait for a final bill but instead of final bill we got the letter from collection agency. 
So we called XXXX again. On XX/XX/2019 XXXX on Monday the XXXX employees didnt like to help us and asked to call collection agency for all information because they have no access to our account. We called twice because during our first phone call the employee gave us too many information and she was even able to access our account and the charges were for some services we didnt use since we have canceled all services 6 days after signing up to XXXX. She disconnected us because she has noticed that she wasnt allow to provide us any information. She disconnected us prior to our second call at XXXX on XX/XX/2019. 
During our second phone call the Supervisor didnt want to provide us any informations, all they could offer was the phone number to collection agency. 
That was very disappointing because not only each time we called XXXX we were promised that XXXX employees will take care about the issue but now even the supervisor is not willing to review the entire situation, she wasnt even willing to listen to us. 
Finally we didnt have any other choice left so we called the collection agency. They told us that we can dispute our situation online. We provided them a receipt from XXXX  and proof of phone delivery to Verizon.They have never contacted us but We were able to log in on their website and we were able to read their message over there. After reviewing our case They returned the case to XXXX. 
So We were glad that finally we will be able to discuss the issue with them. But we were wrong. They send it over to another collection agency. When we called the new Collection agency they were not even able to provide us any information what is the bill for. They told us to call XXXX. But XXXX told us to call the collection agency. 
At that moment It is affecting our credit score and because of coronavirus situation its much harder to dispute and find solutions to this issue. 
We really need help in this situation because it feels like there is no justice. 
We dont understand why do we have to pay someone for something we dont owe and we dont own ... and why does it have to affect us so badly. We are innocent, we work hard and are paying bills on time. XXXX does not care about humans right ... from the very beginning of this story. 
And the new collection agency is not providing any information. The only thing they do is to collect money from us. Even that we do do not owe them and XXXX anything but {$50.00}.