Are there any regulatory requirements regarding the language that must be in a past due notice? i.e. must a phone number or way to contact the institution be on the notice? Must it say "second past due notice" What regulation or law governs past due notices?

I don't think there are any federal regulatory requirements. Many banks use the past due notice to add a sentence about HUD's requirement on real estate loans to advose the Homeowner about counseling centers.

Many banks have the model notice information for the FACT act on past due notices.

This is the Model Notice B-1: "We may report information about your accoun to credit bureaus. Late payments, missed payments, or other defaults on your account may be reflected in your credit report."