APP_PHONECARD.H
#define PC_CARD_NUMBER 13

/* define our structures */
struct pc_phonecard {
	int cardid;
	char number[PC_CARD_NUMBER];
	int credit;
	int ratechangepercent;
	int ratechangestatic;
	int connectcharge;
	int increment;
	int inuse;
	int expire;
	int active;
	char *callerid;
};

IN APP_PHONECARD.C
#include <phonecard/app_phonecard.h>

/* the phone card structure */
struct pc_phonecard phonecard;

MY ERROR:
app_phonecard.c:58: error: storage size of `phonecard' isn't known