First of all I would call the bil and sil and TELL them about the situation and that it is up to them to stop it because it is interfering with your life because of their inability to deal with their debt. They are most likely unaware the calls are happening and will be MAJOR embarrassed. Embarrassed enough to put a stop to it. They in fact have grounds against the company if after they notify them both verbally and in writing if you are ever called again.
NOW a lesson in how to stand on a desk and stop the nonsense. I had to do this when we first moved into this house because the people who had previously had our phone number skipped out on a LOT of bills.
Offer to take the phone name and phone number of the person calling. Once you get those write it down, repeat it back to them. Ask if the call is being recorded. State you want it to be. Then ask for a supervisor. When you get up the food chain a bit (and record all the names you have spoken to going up) tell that person you have spoken to that you have the number and names of the people who evidently don’t understand plain English and that you are going to repeat yourself only one more time. The couple does NOT live here, they have never lived here, and you have ZERO contact with them. That they are calling on a work only phone and are putting your job in danger because you are not allowed any private calls on it.
If they EVER call this number again you will file every possible charge you can against them with both the fair credit act and the local law enforcement agency because obviously they are doing illegal activities or they would not continue to call. Also tell them you will bring a civil lawsuit against them and name all the people on your list as people to sue individually as well as the firm itself. As is your right by the fair credit act and the laws against harassing phone calls. Then ask if this person understands quite clearly what you are telling them? When they say yes, then say,” good because if I am ever called again I am going after you personally. “
It stopped all the calls here dead I never had another one after that.
And in case you are worried it will cause problems for the inlaws, it won’t. The collectors are pond scum and they are seeing you as a mark because you are in some way or another taking their nonsense, at least in their eyes.