There appear to be several issues here. First of all, yes, you are directly liable to the insurance company for any money that is past due. But, second of all, if your employer withheld money from your paycheck that was supposed to go to the insurance company, then the employer would be liable to you for that amount.
Andrew M. Bonderud, Esq. is an attorney with The Bonderud Law Firm, P.A. He offers free consultations 24/7. Andrew's posting here is not to be considered legal advice nor does an attorney-client relationship exist.
Short answer... Yes. Make sure you weren't shorted by the employer. You may be able to negotiate a lower amount with the collection agency because they typically have rom to move.
I agree Mr. Bonderud and Mr. Watkins that your employer needs to account for any money withheld from your pay but not tendered to the insurance company or to whom the payment was assigned. Was the insurance policy a non-cancelable policy? If not, did the insurance company send you a notice cancellation? The papers you signed when you purchased the insurance and when you agreed to the payroll deductions should answer your question about what happens if the insurance premium is not paid.