I didn't even contact anyone in particular- I just mailed the following letter to the address on my credit report... short and sweet... and obviously effective 
NCO Financial, Inc.
PO Box 15636
Wilmington, DE 198505636
Re: Acct # xxxxxxx 01/31/2014
To Whom It May Concern:
I am writing to you in hopes of your help and understanding. I had a past due balance on a medical bill which was turned over to your collection agency. I had an illness and no insurance which left us buried in medical debt, with more bills coming in than we could keep track of; and this one was overlooked and placed with your agency. We have worked very hard to pay all our medical bill balances in full. Even though I have paid the balance to your agency, your collection account remains on my credit files. We are in the process of looking to buy a house, and this collection account, though paid in full, is impacting our ability to do so. I am hoping that as an act of goodwill, your company would consider removing your record from my credit files. This action would help us considerably and allow my family to move forward and have a better future. Thank you in advance, I look forward to your reply.
Thank You,
lmfields
Starting Score: Me:530 Him:519