Why would you think it had been sold?Have you had a letter from AIB telling you that?
If not,most likely DCSI are just acting on behalf of AIB .
If you ring them,they will most likely ask you to fill out a statement of means.Will this show an ability to pay?
If not,offer them 1k in full and final settlement.If they reject,you can always bump it up slightly ,but if you offer the full 2k at first,you've really shot your bolt if they reject it.
EDIT:
Can AIB sell your debt?Yes as long as they tell you.
What DCSI are doing us ratcheting up the pressure with each letter to you seeming direr that the one before.This is a psychological process called Dunning.
It could lead you to crack and offer more for the debt than you can afford,or agree to a repayment schedule that is optimistic,and ultimately fails.