Why are my customers not receiving SMS?

We receive delivery statuses from a mobile operator.

Possible reasons for not receiving SMS, according to the operator:
— Phone memory is full;
— Negative SIM card balance;
— The function of blocking SMS from short numbers is enabled;
— The anti-spam function is enabled;
— Installed apps related to allowing access to SMS, which conflict with the phone operating system and SMS cannot be received;
— Phone failure;
— Incorrect settings of SMS center;
— Message interceptor virus.