PCS ACS or Cash Advance USA Constant phone calls from company to my work; afraid of losing job; want harassment to stop Internet

I thought this debt collection was legitimate until I came across all of these reports that sound exactly like what is happening to me and my sister.  I just spent 30 minutes on the phone with these people.  I asked for something in writing and they faxed me a 2 page form requesting my credit card information and verifying that I owe them $350.  The address they gave was to an apartment in New York.  They say they are a consolidation dept and request that I pay them $350 by today or they will take me to the courthouse for legal process.  They told me once I send back the form they will send me something in writing like a receipt.  That they represent major finance all over the USA.  Gave me the name of Shane Watson which is diffficult to believe because this person barely spoke English.  Heavy India accent.  Sounds like a call center because I can hear a lot of people in the background.  They have called my work number constantly and have jeopardized my sister's job already and brought up her name in today's call.  When I asked questions the person I spoke with laughed to the people there with him and said I will not pay them, like I was ridiculous.  I am also getting these calls that come up on my cell as 000.  How do we get them to stop???  Is there anyone that can help?  Thank you.