the problem is with your program.
unsigned char str1[4]=”ATn”;
unsigned char str3[10]=”AT+CPIN?n”;
unsigned char str4[16]=”ATD7418211136;n”;
You need to use ‘r’ and not ‘n’ . so change that part to
unsigned char str1[4]=”ATr”;
unsigned char str3[10]=”AT+CPIN?r”;
unsigned char str4[16]=”ATD7418211136r”;