IF you had other insurance at the time of service, often times co-insurance claims get delayed for a myriad of reasons...But, they/hosp may have waived their right if THEY delayed the billing KNOWING that you had other insurance. That being said, IF they did NOT know you had other insurance because you forgot/etc to notify them, then you will be stuck paying because the "other" carrier will deny due to delay..
I would call if it is the hospital (NOT if its a debt company), ask them for proof they billed, what they got paid and by whom and any all denials. Follow that with a written letter indicating what they TOLD you
and that you are awaiting the proof for this long delayed billing.
Then await proof, review to see if in order...perhaps the other company is actually at fault for not paying...you need to review it all and figure out what went wrong, if it did. Then
either process it accordingly and by that I mean, pay it if you can outright...OR ask them for a billing plan if you can not.
Good Luck....perhaps you will learn that something went awry and itll get resolved with minimal payout from you/co pay or deduct??