This why you shouldn't send any money to the insurance company.
They had/have a contract with your former employer. Every month they exchange funds depending on the number of employees covered that month, and any other fees. You are not privy to how partial months are addressed, nor are you sure if the company is self insuring so the actual premiums may have fixed part and a flexible part.
The amount that is withheld from your paycheck is an agreement between you and your employer. They can withhold anything from 0% to 100%, and it can even depend on if you have employee only, employee + spouse, or family.
The insurance company has no idea how much should have been taken from your paycheck. If they believe the employer owes them more money, that is not your concern. If the employer failed to withhold money they should have, that is between you and your employer. Therefore the insurance company should not be contacting you...