function FormatPhoneNumber(number_string, format_string) {
    
    format_string_len= format_string.length;
    num_of_x = 0;
    i=format_string_len
    while(i--){
    
        if (format_string.charAt(i) == 'x') {
            num_of_x++;
        }
    }
    
    difference = number_string.length - num_of_x;
    switch(true){
        case (difference == 0):
            
            break;
        case (difference > 0):
            
            number_string = number_string.slice(difference);
            break;
        case (difference < 0):
            
            return number_string;
    }
    
    // The wrapping if/else statement is not necessary anymore so I removed it.

    formatted_string = "";
    pos = 0;
    for (i = 0; i < format_string_len; i++) {
        if (format_string.charAt(i) == 'x') {
            formatted_string += number_string.charAt(pos);
            pos++;
        }
        else {
            formatted_string += format_string.charAt(i);
        }
    }
    return formatted_string;
}