Did you get an iMessage activation error message on your iPhone or iPad? This usually occurs when you update your iOS version or when you move abroad and change cellular networks. There are several ways to fix this problem.
Method 1: Check your device settings. Make sure your device is connected to mobile data or WiFi. You also have to activate iMessage with your phone number. SMS charges apply.
Method 2: Disable iMessage and reboot. Go to Settings>Messages and deactivate iMessage. Restart your device, then enable iMessage again.
Method 3: Activate the service while on Airplane mode. Go to Settings>Messages, then turn off iMessage. Put your device in Airplane mode, then turn on the WiFi. When you connect to a network, click on the blue arrow, then delete the DNS and type in 18.104.22.168. After that, go back to settings. Turn iMessage back on, then turn off Airplane mode. You will get an alert that says “Your carrier may charge for SMS”, select Cancel. The iMessage activation error message should disappear after this.