It’s not illegal, at most it’s inappropriate behavior.

After the credit card repayment is overdue, the bank will use various methods to collect the payment:

Generally, when the credit card payment is overdue, the bank will make a phone call to collect the payment.

Overdue payment If the debt is not repaid for a period of time, the bank's credit card center staff will come to collect the payment, including some staff may use stickers and other methods.

If the debt is seriously overdue, the bank will sue the debtor and bring him into custody. Blacklisted, you will become persona non grata in the bank in the future, and other business transactions will be affected.