What is the date the payment is due? If it is the first of the month, then of course they will call you after that to tell you that your payments are late, because THEY ARE!
There is no such thing as a Grace Period. Payment is due by the date indicated. Not within 2 weeks past the date due.
What you are saying is that most of the time you make late payments, but on 2 occasions, the payments were more than 2 weeks late.
Novel idea here - if you want them to stop calling you to let you know you are late on your payments then PAY ON TIME.