Product: Debt collection

In summary here are the sequences of events : XX/XX/XXXX I visited XXXX XXXX and received a treatment plan for a XXXX and paid that day {$1000.00}. 
XX/XX/XXXX We outlined treatment plan further and I signed up for payment plan. 
Early XX/XX/XXXX Between XXXX and XXXX the XXXX procedure was performed and I went in for adjustments to get it just right and it never did really fit. Next, I was no longer a patient of XXXX XXXX after further requesting adjustments to the XXXX. It just never did sit right and we have documentation here including professional assessment of the work. 
Mid XX/XX/XXXX I needed to get my XXXX corrected. I visited a second dental office XXXX XXXX XXXX for advice on how to get the XXXX correct. I was quoted {$8700.00}. 
Seeking a more affordable option I found a third dentist XXXX XXXX XXXX, XXXX, to fix the XXXX for {$5700.00}. We proceeded with the work and you will find the records attached. I paid the full {$5700.00} and my dental conditions are back in order. 
Please note that after XX/XX/XXXX I was never sent a bill from XXXX XXXX and I never had any automatic payments taken out as was outlined in the payment plan in XXXX of XXXX. 
My biggest concern is the letter I received on XX/XX/XXXX from Wakefield & Associates with a bill from XXXX XXXX for {$2500.00} and an additional charge of interest of {$150.00} totaling XXXX due. 
I am concerned about my credit and do not feel I owe XXXXXXXX XXXX   this amount. I went to their in person office and expressed my concern and they said there is nothing they could do that I needed to pay the bill. 
so here are my concerns/complaint to XXXX dental and Wakefield associates : 1XXXX XXXX did not complete the service ( that I contracted with them ) in timely manner, professionally, sufficiently and to my satisfaction and dismissed me without a valid reason in the middle of treatment without providing me any referrals/sources to continue my treatment. 
2- I was never sent a bill from XXXX XXXX and I never had any automatic payments taken out as was outlined in the payment plan and I was sent directly to the collection agency. 
3-XXXX XXXX did not bill my insurance company and now is looking after me to even pay for the amount that insurance company could and should simply pay. 
4-I disputed the bill with Wakefield collection agency via phone, mail and online on their website and they are still calling me that my account is not disputed and still active and they are threatening me that if I do not pay the bill it did and will continue hurting my credit score and they also sending me a validation of the debt which is included the treatment service contract which is not legit because XXXX XXXX do not have my signature on it which looks like XXXX XXXX did modify the documents and the bills on their end without my acknowledge for their own profit. 
5-Collection agency still calling me and offering me discounts to pay the full amount however I keep telling them that i should not pay the amount and they threaten me that if i do not pay the amount they will keep charging me interest and it did and will hurt my credit score. 

Please review attached documents and I hope to have some help in this matter.