Since you are an iphone user there is a way to use your iPad to text to non iMessage devices. Make sure your iPad is turned on. Now your iPhone go to Settings: Messages. Tap on Text Message Forwarding. Move the slider to activate forwarding. A code will be displayed on your iPad. Enter that same code on your iPhone. That should allow you to send / receive non iMessages on your iPad.
The iPad does not have a phone number associated with it. My understanding is that SMS messages go through the voice network which the iPad has no capability of reaching.