Since the launch of iOS 5, Apple has supported texting and IMs via iMessage. Sending an iMessage is a lot like sending a text message, except that it uses the phone’s Internet data connection (or Wi-Fi) to send the message. For people who are on limited text messaging plans, this is a great way to keep that message count low. The setup is supposed to be automatic; however, people have been complaining ever since it launched about receiving a “Waiting for Activation” error from iMessage.
Let’s cover a few steps to try to resolve this conflict.
Disable and Re-Enabled iMessage
Open the Settings app on the iOS device and choose Messages.
Toggle the switch next to iMessage to the OFF position. Wait at least 24 hours.
Follow these same steps to turn iMessage back on.
Restore the iPhone from the Computer and iTunes
Plugging in the iPhone to the computer should automatically launch iTunes. If it does not, make sure iTunes is running.
Select the iPhone from the menu on the left, and choose the Summary tab.
Then select Restore iPhone.
iTunes will then prompt the user to make sure the restore is okay to perform, and warn the user that all data will be lost. Click Restore to continue.
Once completed, the phone will need to perform the initial setup again. As long as the user’s data was backed up to the iCloud of the computer, restoring should be pretty straightforward with no loss of data. The restore will download the latest firmware, which has resolved the issue for many of the people suffering from the problem.
Variation of the iPhone Restore
If the above step does not solve the problem, some suggest that the user attempt the restore, but not log into iTunes or iCloud.
After restoring the phone, the phone will ask the user to log into iTunes. Click the skip button.
The phone will then ask the user to log into iCloud. Also click the skip button.
Verify that iMessage works by sending an iMessage to someone with an iOS device (preferably a phone).
If the message is sent and there is no iMessage activation error, then connect the phone to an iTunes account by choosing iTunes & App Stores from the Settings app.
Then there will be an option to Sign In. Click that and follow the directions.
Next, if the user chooses to, select iCloud and perform the same procedure to Sign In.
If neither of these solutions solves the problem, be sure to check out this thread on the Apple Support Communities for more tips. Good luck!