US Cellular Someone threatening to garnish my wages stating they are us cellular and they they were going to report it to the IRS Internet

My sister was contacted by this law firm and the assistant to a lawyer said that I owed US Cellular $700 but with the interest that has occured since the bill in early 2000', s I'm assuming.  However they could not provide me with any information as far as was this a bill I had or someone else opened in my name.  I went on to as them to verify my social security number and that could not provide this and anyone who ever has had any form of contract phone nows you need a social security number to start the account.  I played along and gave them a old netspend debit card that I no longer use and they faithful try to deduct money on the 20th of each month for almost six months now, im trying to see if this is a real situation that I should tae care of or a scam to get me to pay for something that isn't mine  If anyone has any information regarding this please let me know what your experinece was or is now after dealing with these characters.