Can international Phone numbers also send in their messages by sending a text message to a shortcode?

This is not possible. You can only send text messages to a shortcode (normally a 4 or 5 digit number) with a wireless carrier which is from the same country as the shortcode is from. The shortcode will not receive messages from phones which have a foreign wireless carrier. If you want to use the product(s) with a international audience you have you to use the international 10 digit number. This will be clear when making you activationcode.