I have a new iPad air wifi no cellular and set up iMessage but I cannot send messages to non iPad or iPhone users from Messages. It always tries to send the message but every time, it just stops and outputs that it has not been delivered. When I turn iMessage off every time I want to send a text message it asks to login. I can send messages on my iPhone to the same people that I want to message to using my iPad, with no problem.
iMessage is actually an instant messaging service, like Google Talk or AOL Instant Messenger, except iMessage is made by Apple and is only for Apple products. Only an iPhone with cellular can send SMS messages as well, and can automatically associate phone numbers of registered iMessage users with iMessage accounts, so if you try to use the app to send a text to a phone number registered to an iPhone, it will send an iMessage instead. iPads cannot send SMS messages to non-Apple devices nor an iMessage to a phone number not registered with iMessage from a Wi-Fi-only device.
Actually, you can send SMS from your iPad to anyone, just like your iPhone can text non-iPhone users. It's a complicated multistep process to set it up, but here goes:
First, make sure you're signed into iMessage on your iPad in Settings > Messages > iMessage.
Jump over to your iPhone and confirm you're signed into iMessage and iCloud with the same account here, too.
Still on your iPhone, go to Settings > Messages > Text Message Forwarding.
Inside Text Message Forwarding, flip the switches to enable sending text messages on one device at a time. Then look at that device – you'll see a confirmation number that you'll have to enter on your iPhone. This ensures you've got physical access to both devices at the time you're setting it up.
Confirm the link by typing the code and you're finished!
iMessage sends the message through Apple to the intended recipient. Because the iPad is WiFi without cellular [data], SIM cards cannot be inserted and therefore the iPad is not able to message non-Apple users. As noted, 'only sim-cards have access to the GSM network to send SMS messages'.
Update: You can now send text messages through your iPad if you have an iPhone. (See here.)
After 3 visits to an Apple store Genius Bar, I was finally informed that Ipad Air messaging only works when sending or receiving text messages with other apple products. This, in my opinion, makes it essentially useless. Since most of my messaging contacts do not have apple products, I turned off my iPad Air messaging and simply use my iPhone for all of my text messaging. I consider this to be a design flaw in iPad Air which will hopefully be recognized and corrected sometime in the near future.