You don't directly call a .c file. If you have functions defined in an external file, you create a header file containing the function prototype. This header file is included in any file that uses the function (and in the .c file that defines the function). The compiler and linker take care of the rest.
The only hitch is that you'll get compiler errors if the same header file is included multiple times. You need a preprocessor directive to prevent this. What you choose to call it doesn't matter, but it must be unique. This is how it looks:
/* header_file.h */
/* function prototypes here */
This is what the file containing the function definition might look like: