Fun facts:

When still known as Academy Collection Service, this agency in November 2008 was spanked for $2.25 million by the FTC for chronic violation of debtors' rights. This remains among the highest penalties ever handed to a debt collector. Shortly thereafter ACS rebranded as Monarch Recovery Management. It did not take them long to attract a new batch of Fair Debt lawsuits since then. ACS or MRM or whatever name it likes needs to be sued by each of you it decides to abuse.

Nationwide Debt Collector Will Pay $2.25 Million to Settle FTC Charges [press release]

Official-type material on US federal collection law:
 Oct 03rd, 2012
Comments on WhoCallsMe cite debt collection agency Monarch Recovery Management. You're likely being used to shame your ex-SIL to come out of hiding, while MRM already has her current phone number. But they will call this skip tracing anyway. An agency rep can be rude and demanding to a point, but legally cannot abuse or harass you.

My advice for a ''not me'' case like yourself can be found here, in a comment from 29 August 2012:
 Oct 03rd, 2012
got a call for ex sister in law from a Paula Cochran she was rude demanded answers and got mad when I told her to piss off and refused to help locate my ex sister in law so who is this?
 Oct 02nd, 2012