Legal Question in Real Estate Law in California

I'd say you are mis-using the grace period. Your loan payment is due before the beginning of the grace period; during the grace period it is late; by the lender's "grace" you are spared a late fee, but the payment is nonetheless late. Think of a Catholic who can confess a sin and obtain absolution. Nevertheless, a sin. To stop the calls, pay by the due date.

Whipple is exactly right on this one. Institutional lenders manage massive cash flows. With all the people paying later and later these days, it is messing with cash flows, especially when people like you always wait to the end of the grace period. So they have chosen to respond to you messing with their cashflows by messing with your phone. You are late after the due date and they are completely within their rights to remind you of that. Be glad they don't eliminate the grace period on you. They have the right to do that too. In fact your loan documents probably provide that habitual lates constitute default.

I agree. The grace period is the period of time for you to make the payment without triggering the acceleration clause, going into default, or apparently incurring late fees. But you are still late during the grace period.