We have been paying on a big IRS tax debt with an installment agreement for over three years. I just got a CP522 notice from their Memphis office, along with a Form 433-F. They're directing to me to fill out the form and call them no later than August 2nd. But they specifically say "Don't mail us this firm. You must call for this review." What should I do to make sure the IRS doesn't raise our monthly payment or cancel our agreement?