A simple program that will convert a string of numbers to the word equivalent. For example, if the user enters 12345 the program will output One, Two, Three, Four, Five. Be careful with this, there isn't a check to ensure the input buffer doesn't overflow.

phone.c

#include <stdio.h>
#include <string.h>

#define CHAR_TO_INT(c) (c - 0x30)

int IsValidPhoneNumber(char *phoneNumber, int length)
{
	for (int i = 0; i < length; i++)
		if (CHAR_TO_INT(phoneNumber[i]) < 0 || CHAR_TO_INT(phoneNumber[i]) > 9)
			return -1;
	return 0;
}

int PhoneNumberToString(char *phoneNumber, int length)
{
	const char *numberWords[10] = { "Zero", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine" };
	int outBuffIndex = 0;

	// make sure that the phone number the user entered is valid
	if (IsValidPhoneNumber(phoneNumber, length) != 0)
		return -1;

	// print out the word equivalent for each digit in the phone number
	for (int i = 0; i < length; i++)
		printf("%s, ", numberWords[CHAR_TO_INT(phoneNumber[i])]);

	// remove the the trailing ", "
	printf("\b\b \n");

	// it was successful
	return 0;
}

int main()
{
	char phoneNumber[20] = { 0 };

	// get the phone number from the user
	printf("Phone Number: ");
	scanf("%s", phoneNumber);
	
	// if the conversion fails, then the phone number was invalid
	if (PhoneNumberToString(phoneNumber, strlen(phoneNumber)) == -1)
		printf("Invalid phone number.\n");
	
	return 0;
}