Undefined reference to 'function name' - c

Question

I am a CS student currently trying to learn C. I just started creating my own header files and as a fun little project I wanted to create a library of my own that can handle big integers in a string based way. There are simply three files; bignumbers.h, bignumbers.c, test.c .
bignumbers.h:
#ifndef BIGNUMBERS_H
#define BIGNUMBERS_H
//Typedef Declaration
typedef struct big_number{
char digit;
struct big_number *next_digit;
} *BIGNUMBER;
//Function Declaration
int number_length(char number[]);
BIGNUMBER create_digit(char digit);
BIGNUMBER create_number(char number[]);
//Macro Declaration
#endif // BIGNUMBERS
bignumbers.c
#include <stdlib.h>
#include <stdio.h>
//Actual functions
#include "bignumbers.h"
int number_length(char number[]){
/*code to get string length*/
}
BIGNUMBER create_digit(char digit_){
BIGNUMBER number_digit;
number_digit = malloc(sizeof(BIGNUMBER));
if(number_digit == NULL){
printf("Memory allocation failed!");
exit(EXIT_FAILURE);
}
number_digit->digit = digit_;
}
BIGNUMBER create_number(char number[]){
}
test.c:
#include <stdio.h>
#include <stdlib.h>
#include "bignumbers.h"
int main(){
number_length("1");
}
Trying to compile test.c gives me an error with the message "undefined reference to 'number_length'
error: ld returned 1 exit status"
There is no typo in test.c as far as I'm aware. So what might be the problem here? I understand that "ld returned 1 exit status" is a linker problem but I don't really see what can cause that.