I received a collection letter for a medical bill I failed to submit to the insurance company, how should I proceed?

Yes. You should document everything that you do. Submit the bill to your carrier in writing. Dispute the debt in writing. Most likely the medical provider erred and failed to submit it. Sometimes the collector claims that it is your responsibility, but if the provider undertakes to submit it and fails to do so properly, that is not correct. Check you credit report and make sure it is not reported.