possible eight-letter combinations

// this code should generate all possible eight-letter combinations from an eight-digit phone number. The procedure doesn't check if the phone number contains 0's or 1's.

Code:

#include<iostream>
using namespace std;

const char* letters[] = {
    "ABC",
    "DEF",
    "GHI",
    "JKL",
    "MNO",
    "PRS",
    "TUV",
    "WXY"
};
FILE* fp;
char str1[15];

void GenCombinations (
    const char* number,
    char str[]) 
{
    if (*number == '\0')
    {
	fwrite(str - 8, 1, 8, fp);
	cout<<str<<endl;
	fwrite("\r\n", 1, 2, fp);
	return;
    }
    else if (*number == '-')
	GenCombinations(number + 1, str);
    else {
	const int group = *number - '0' - 2;
	for (int i = 0; i < 3; i++)
	{
	    *str = letters[group][i];
	    cout<<str<<endl;
	    GenCombinations(number + 1, str + 1);
	}
    }
}
int main(){
    char str2[]="42472888";
    strcat("\0",str2);
    GenCombinations(str2,str1);
     cout<<str1<<endl;
       system("PAUSE");
    return 0;
    }

Please dont cross post

Replies - i dont know what is wrong with it (help plz)