First NEVER talk to these "people" on the phone as they will try and get you to agree to anything.
Before you even think about making any sort of acknowledgement or payment to a DCA you MUST ensure that they have the LEGAL right to collect the debt.
This is achieved by requesting a copy of the Signed, Executed Credit Agreement via a Consumer Credit Act (CCA) request.
There is a template letter to be found here:
Creditors and DCAs - Letter Templates & Budget Planner
Letter N.
Is it important to start the letter:
I DO NOT ACKNOWLEDGE ANY DEBT TO YOUR COMPANY in Big Bold letters.
DO NOT sign it, but use a handwriting font instead. There are many to be found here:
Free Handwriting fonts
It is best to send this request via recorded/special delivery as there are some important deadlines to observe.
After 12 WORKING days the "debt" is in default and stays that way until the request is complied with.
If a FURTHER month passes then the DCA has committed a summary criminal offence and the matter should be referred to Trading Standards for action once the DCA continues to demand payment.
I hope this clarifies some things.