unpaid doctor bill-now in collections

[stew]

I have been contacted by a collection agency about a bill that is 5 years old. My wife went to a doctor on 8/26/02. At this time we had dual insurance coverage. She paid the co-pay at the time of service. The doctors office was to handle the billing of both insurance co. Apparently the billing service that the doctors office was using failed to submit a bunch of bills so there was no payment from the ins companys. We never received any notification about this or we would have submitted the claims ourself.--Now in 2007 a "new" billing service has taken over for the doctor and must have discovered all these unpaid claims---they submitted the bills to our ins co. on 1/11/2007 for service date of 8/26/02--our primary ins co responed-claim does not meet filing deadline. this has been turned over to a collection agency--charging 500.00 for principal and 225.00 for intrest. Are we liable for this due to the fact the billing service never filed to the ins co, never notified anyone about the unpaid bills, and now the ins won't pay because the service was 5 years ago.