http://www.creditinfocenter.com/rebuild/debt_validation_workflow.shtml
The 1st step you need to do is go to your credit reports per the 1st poster. This if free and can see with all 3 credit reporting agencies, if this debt is listed, which most likely is.
Even though it has been 3 1/2 years since you got this service, you might dispute this if they are not legally able to collect. Read the site attached and it explains how. Due to the length of time, sounds like this debt may have been sold to many collectors over the years. With each collector, they pay less than the one before. Many times to get the "original" contact or bill, they must pay more for that and many prefer to try and collect w/out.
And yes they have up to 7 years to collect. But here is something most people don't understand, that when you pay ANY money, the 7 years start over again to collect. This usually happens after the 7 years has run and they get you to pay "something" to start the clock all over again, since over 7 years, they are barred.
So right now, they want to get this paid and you maybe able to negotiate this, like the other poster for 50%. So say for instance you settle for $900 vs the $1,800, then they will send you at the end of the year, a 1099? which means that you would have to add to your tax returns that you in fact got $800 more in income and pay taxes.
Have you moved a lot, changed telephones a-lot over the last 4 years? My step daughter is an absolute mess with handling money and is constantly being pursued into collections. What has made it difficult for the debt collectors with her, is that she has married twice in 5 years, so had 2 married names, went back to her single name and has also moved to 3 different states, plus has changed her phone number around 6 times during that time. We keep getting phone calls at our house (her father) for these debts, since they have no clue how to find her. They keep hoping we will answer the phone and give them info, which we wont, since many of these knew by her credit it was lousy, but then loaned her the money, so they are the idiots.
Do your research 1st, after you have done so, then you might have to actually pay them, but if you do, try your best to negotiate it. There are sites that tell you how.
good luck