I am writing a code which prompt user to enter her mobile number and in result..I code to display it's user SIM network according to some principles which I code but I don't understand whats wrong with my code.. It saves data in a text file...

// here is my code
#include <iostream>
#include <ctype.h>
#include <string.h>
#include <fstream>
#include <cstring>
#define MAX 15
using namespace std;
char fourdigOfnum(char *s1,char *s2) {
    int i=0;
    while(i<4)
    {
        s1[i]=s2[i];
        ++i;
    }
    s1[i]='\0';
}
int main() {
    char num_dig[4],simCard[MAX];
    char mobile_num[MAX];
    ofstream file;
    file.open("mobile.txt",ios::out);

    cout<<"Enter a Mobile-Number : "; cin>>mobile_num;
    if(mobile_num[0]==0||strlen(mobile_num)==11)
    {
        //num_dig=fourdigOfnum(str_num);
        fourdigOfnum(num_dig,mobile_num);
        if(num_dig[strlen(num_dig-1)]==0)
        {
            strncpy(simCard,"Jazz",sizeof(simCard));
        }
        else if(num_dig[strlen(num_dig-1)]==1)
        {
            strncpy(simCard,"Zong",sizeof(simCard));
        }
        else if(num_dig[strlen(num_dig-1)]==2)
        {
            strncpy(simCard,"Warid",sizeof(simCard));
        }
        else if(num_dig[strlen(num_dig-1)]==3)
        {
            strncpy(simCard,"Ufone",sizeof(simCard));
        }
        else if(num_dig[strlen(num_dig-1)]==4)
        {
            strncpy(simCard,"Telenor",sizeof(simCard));

        }
    } else {
        cout<<"You entered a Invalid number "<<endl;
    } 

    if(file.fail())
    {
        cout<<"Sorry File is failed to open "<<endl;
    } else {
        file<<"Your Mobile-Number is "<<mobile_num<<endl;
        file<<"This is a "<<simCard<<" number "<<endl;
    }
    file.close();

    system("pause");
    return 0;

}

I think the lack of replies means you have to tell more about what's broken. If possible try to code it up on IDEONE.
https://ideone.com/Vc0043 coughs up an error on the line system() so you also may have to reveal your compiler setup.