What to do if a collection agency found out the phone numbers of a friend and my ex husband and called them looking for me?

A collection agency can call anyone to look for you as long as they don't disclose private matters, namely that you owe "x" amount of dollars.  If they do discuss your private matters with third parties, then you can file a complaint with your attorney general's consumer affairs division.

With regard to the gathering of phone numbers-- in the age of information technology, a ton of information is displayed on the internet and sold by companies.  This is why you get notices from your financial institutions that they "do not resell private information."  But unfortunately, many entities do.  Every time you enter a contest or apply for credit-- there is a potential that your information will be resold to telemarketers or collectors.  Every time your ex- enters something or applies for credit--his data will be redistributed.  There are actually companies that routinely collect and sell data reports that will list every neighbor you have ever had, everyone you've been married to, and all the cars you've ever driven. Once you are cross-referenced with someone (like your ex-), then both of your names will show up on the report forever. Most of their info comes from public records-- while some will browse social networking sites picking up information here and there.  Then... collection agencies purchase these reports... and you get the call.  The collection of this data is legal in the United States... even though somewhat disturbing.

You mention a sub-issue-- which was they wouldn't tell you how they got the numbers or "what this was in regards to."  A collection agency is not required to tell you how they found you.  The person calling you is probably a cold-call, flunky on the phone and really doesn't know.  However, if they are calling you and demand that you pay on an account, they are required by federal law to tell you the nature of the debt and who it is alleged owed to.  If you do not agree that you owe the debt, then you have the right to dispute the debt and they cannot resume collection activities until they have provided you with verification of the debt.  If they insist that you to pay money without verification, they could just be a scam agency.  Some will call you and say they that you owe a debt-- and in order to verify whether it's you or not-- they will need your social and DL.  Once you give the social and DL they quit calling-- but then someone is using your identity within a few months.  If these people ask for your identifiers, tell them to produce their verification first.  If they continue to harass you, request that they send you something in writing regarding the debt and who they are.  From there, you can have an attorney send them a more specific demand letter.  Often, once they get an attorney demand letter (which won't cost you much), they give up the gimmick.