iMessage only works on cellular data, it won't work on wifi only.

Hello there, Andres_F.

The following Knowledge Base article is great for troubleshooting the issue you're describing:

iOS: Troubleshooting Messages

http://support.apple.com/kb/ts2755

If the first steps dont help you get taken care of, the section that follows under "If you still can't send or receive iMessages":

If you still can't send or receive iMessages

1. Ensure that iMessage is turned on in Settings > Messages.
2. To send and receive iMessages, your device must have a valid cellular or Wi-Fi data connection.
   • iPhone: Troubleshooting Cellular data
   • iOS: Troubleshooting Wi-Fi networks and connections
3. iMessage registration validates your Apple ID for use with iMessage. If you are unable to activate iMessage, follow this article.
4. If an iMessage cannot be sent, it will be resent using SMS or MMS, depending on the content. You can turn this behavior on or off in Settings > Messages > Send As SMS. Carrier messaging rates may apply.
5. Verify that the Apple ID or phone number is listed in Settings > Messages > Send & Receive.

Note: To receive iMessages sent to your phone number on your iPad, iPod touch, or Mac, you must meet the following requirements:

• iPad or iPod touch using iOS 6 or a Mac using OS X 10.8.2.
• Verify that you are signed in to your Apple ID account on Settings > Messages > Send & Receive.

Thanks for reaching out to Apple Support Communities.

Cheers,

Pedro D.