You have two issues: Handling the harassing calls and correcting the late payments.
Collection harassment calls
Previously, I discussed what a consumer can do when a creditor uses harassing behavior (see Advice If You’re Being Harassed by Collection Agent). Under the Fair Debt Collection Practices Act (FDCPA) creditors are not permitted to call you at work if you ask them to stop.
You stated you asked GMAC to call your cell phone and cease calling your work telephone number. You are not avoiding their calls. Since GMAC has not adhered to your verbal requests, send GMAC a return-receipt letter stating that your job is at risk because of their calls. Reiterate your willingness to have them call your cell phone or write you. If, after your letter has been received, and they continue to call your employer, file a complaint with the Federal Trade Commission (FTC), the federal agency responsible for enforcing the FDCPA.
Though you do not mention what will happen to your credit score if you continue to make payments late, I want to give you some information on that subject. Below are links to resources you may wish to review to see what happens to your credit score if your payments are late or delinquent, what will happen in you allow a repossession, and what you can do to resolve any resulting debt.
Write a letter to GMAC regarding the collection harassment as soon as possible. Also, call GMAC and reiterate that their calls are causing issues at work and you could lose your employment which, in turn, could result in no income to pay for the loan. I would also see if you can have GMAC change the date the payment is owed so that you will not be late in making future payments.
I hope this information helps you Find. Learn & Save.