Billed by a third party out-of-network vendor. How is this even legal?

I went to an in-network cardiologist in an in-network hospital. He prescribed a stress test and a holter monitor to be worn for a month. I went to the department that does both the stress test and gives out holter monitors in the same in-network hospital. I received my bill and paid it right away. My insurance was then billed by a third party vendor I’ve never heard of and upon calling my insurance, it turned out they’re out-of-network. I found out that the third party vendor is the supplier of the Holter monitor. I called the hospital billing department and they said I am in the clear with them with all bills paid. They couldn’t tell me how it is that I am billed again because I have zero balance with them.

How is it that this third party vendor can bill my insurance? Is this even legal? When I went to the department to pick up the holter monitor I wasn’t given any form to sign that stated that I may be billed by an out-of-network service/vendor. I called the number provided on the invoice of the third party vendor; it seemed like a small office with one billing person and one office manager. I was told that they will call me back because their billing person was busy when I called, but they have yet to call me back. It’s past 24hr since I called. I am very confused how this works and makes me question the legality of this absurd operation. Also it makes me worried that if I don’t pay, they can ruin my credit score.

For clarity: I am located in Massachusetts and am in my 30s