Product: Debt collection

We were clients of a XXXX hard line telephone service for ( 40 years ) and Internet service providers for over 12+ years. We no longer wanted a hard line phone as we did n't use it, it was a total annoyance, and my wife and I both had mobile phones. 
I got in touch with XXXX and explained to them my desire to discontinue our telephone service. ( I 've had this telephone for over 40 years. ) XXXX said that they could remove the phone line but they would only credit our monthly service bill for only {$4.00}. Our bills indicated that XXXX was charging us {$30.00} per month for just the telephone service. Due to length of time we were clients, we were no longer under any kind of service contract with them. We decided that XXXX was only interested in bilking us for service we no longer wanted or needed. They were also providing us with a very slow internet speed, a service they no longer, in fact, provided to new clients because, " It was too slow ''. My wife and I decided that it was time to leave XXXX because we no longer felt they were being ethical or reasonable in their business dealings with us. We found another XXXX who would provide excellent service, at a charge far less than XXXX, and treat us with respect and honesty. We ceased using XXXX services on XX/XX/2017. 
I called XXXX to discontinue service with them and return their modem, which I did immediately. At the very end of my conversation with the service representative to close our account he told me that XXXX would charge us {$150.00}. for a disconnection fee. I have been with the company for over 12+ years, we did not have a current service contract with XXXX , nor any written or verbal agreement that there would be any disconnection fees applied to us whatsoever. I asked XXXX to provide proof of this " contract ''. They never provided proof of a contract as they promised to do. Is this their way of conducting business with long-term clients? 
XXXX followed up by only sending us a non-itemized bill in an attempt to bury this fraudulent charge inside our final bill, without ever providing us with any proof of our responsibility for such a contract charge. They are not being ethical in dealing with us. 
They have not sent the proof we requested, but instead have turned the matter over to a collection agency. We absolutely dispute that after over 12 years service we were still under a " contract ''. This is a false claim on their part. Plus they are not furnishing us with a bill that itemizes and dates all charges. I find this a rather unsavory way to conduct business, especially with a long-term client. 
Yesterday, we received a letter from a collection agency working for XXXX without any itemization or explanation of charges. Just a final bill amount, without any documentation, nor " proof '' of a " contract '' which XXXX claims we had, but will not show us. How can we know what we are being charged for? We were also informed by a XXXX telephone agent over 6 months ago that we were " no longer under any contract with them currently ''. 

How do we go about having this {$150.00}. unsubstantiated charge for disconnection by XXXX dismissed? XXXX refuses to deal with this matter in a civilized way. 
They are demanding payment for something that they have not provided evidence of. This is unprofessional and arrogant on their part. We have always dealt with them in an ethical manner, they are not responding in kind. 
In my research on XXXX on the web I 've noticed that XXXX has a multitude of lawsuits against them for their poor business practices all over the USA. It appears as though I am not alone in my complaint against them for unprincipled business practices. They appear to be too big and powerful, that they feel the law does not apply to them. We need more choice, not more mergers! Competition will provide for better service. A