ID themselves as Yale New Haven Health, deny they are debt collectors. But of course that's exactly what they are. News reports of a few years back show members of this company owning a large stake in a New Haven debt collection company.
Yale New Haven Health, see:
https://www.ynhh.org/ynhhs/billing-insurance.aspx
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If they are calling you about a bill, always demand they send you a copy of the bill by US mail. If they have not already done so.