" Syntax error near unexpected token `636' "

Execute this way:

./a.out "(636)394-6659"

Or as you don't check arguments, this way is the same:

./a.out

You don't use argc or argv, so use this prototype for main()

int main()

Also, you need to consider string a null terminated arrays of chars.

char leftBrack [2], rightBrack [2], dash[2];

strcmp() does not do what you believe it does. You need to use strncmp with a third argument equal to 1. Also atoi() checks the whole string, not a mere character, just check token[i] - '0' is a number between 0 and 9.

Fixed code:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#include <assert.h>

/*
   Parameters
   token - A string

    Return:
    1 if the string is a valid phone number,
    0 otherwise

    A valid phone number takes the form
    (xxx)xxx-xxxx where x is a digit 0-9

*/

int isPhoneNumber(char* token);

int main()
{ 
    //This is a valid phone number
    assert(isPhoneNumber("(123)456-7890") == 1);

    //These are not valid phone numbers
    assert(isPhoneNumber("123-456-7890") == 0);
    assert(isPhoneNumber("1234567890") == 0);
    assert(isPhoneNumber("(123) 456-7890") == 0);
    assert(isPhoneNumber("Hello World") == 0);
    assert(isPhoneNumber("") == 0);

    printf("Congratulations! You finished the prelab!");
    return 0;
}

int isPhoneNumber(char* token)
{
    int i, digit;
    char leftBrack [2], rightBrack [2], dash[2];
    strcpy(leftBrack, "(");
    strcpy(rightBrack, ")");
    strcpy(dash, "-");
    for (i=0;i<13;i++)
    {
        switch (i)
        {
        case 0:
            if (strncmp(token + i, leftBrack, 1) != 0)
                return 0;
            break;

        case 4:
            if (strncmp(token + i, rightBrack, 1) != 0)
                return 0;
            break;

        case 8:
            if (strncmp(token + i, dash, 1) != 0)
                return 0;
            break;

        default:
            digit = token[i] - '0';
            if (digit < 0 || digit > 9)
                return 0;
        }
    }
    return 1;
}