United Processing harassing us--the family

Back in August of this year we kept getting calls from United Processing looking for my sister in law. We were shocked that they had found us given 1.) we live about 2000 miles away from her, and 2.) she has a different last name than we do (she's married). They left several messages saying they were going to arrest SIL if we (or her) didn't call them back, that it was a Federal indictment, etc. and so forth. At that time my MIL and FIL were visiting, so they finally got ahold of SIL and I guess she called them. The calls stopped.

This morning at 7:09 a.m. United Processing called and woke me up. The guy asked for my husband, who wasn't home, and then proceeded to tell me he was looking for DH's sister. I explained that we hadn't had contact with her in over 7 yrs. and to please remove us from his list. He became irate, started yelling, said he would call me EVERY 15 mins. ALL day until we told him where SIL was. He said it was his right to harass me. I said if he did that I would have my attorney contact him. Thus far he has not called back but I doubt this is the last we've heard from him. It is the same guy who called us and harassed us back in Aug.

I want to know what we can do to stop the calls? We are not the ones they are looking for, we do not talk to SIL, and he has no right to call us at 7 a.m. or EVER and treat me the way he did. I am not okay with just letting the calls to go voicemail--I want them stopped. Is there a way to do this short of paying my attorney to write a cease and desist letter?

Thanks in advance!

P.S. They are also harassing my MIL and FIL as well and I'd like to help them if possible.