iPad air - text messages won't send
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.

http://support.apple.com/kb/HT3529