Calvary Portfolio Services (Calvary Collection Agency) Calvary Portfolio Collecting Debts For Sprint Customers Hawthorne New York

NEED SOME HELP PLZ!!!!!

I've been getting calls from calvary.
I finally got to talk to them today and first, they were confirming my SSN and my addresses. I asked them how they got my SSN and they said Sprint has provided them with such information.

Anyway, they told me that I have a Sprint account and owe about $350 debt. Well this is partially true. I happened to 'co-sign' for somebody I know when he was making a new Sprint account. He needed someone to sign up because back in 2004, he was under 18. The employee of Sprint told me that my information will be removed from my friend's Sprint account as he becomes over 18. Unfortunately I did provide my SSN under his account (Calvary says I'm the primary account holder, which shouldn't be), and I believe they are calling me because this friend I know didn't do a good job on paying his bills. I told Calvary rep. exactly what happened and the rep told me to contact the friend and call them back again.

So here are some questions I have:

-My friend is willing pay his debt of $350, but should I contact Calvary again and make payment and follow other procedures that they told me to do?
-Should I contact Sprint?
-I'm worried about identity fraud. How can I make sure that they will not use my information on other purposes.
-I DO NOT want my credit score to hurt from this. Is there any way I can get it back to previous score?
-In worst cases if I need to file a report or contact a lawyer, what is the procedure? Also, does it cost a lot?

George
Palisades Park, New Jersey
U.S.A.