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If you are having an issue with iMessages on your iPad I suggest following the steps in the following article. The troubleshooting steps refer to iPhone but the steps to isolate and resolve this issue are the same. If you iPad is Wi-Fi only and not cellular you can skip the steps that have you troubleshoot the cellular connection.
To resolve issues with sending and receiving MMS, follow these steps
- Go to Settings and turn airplane mode off.
- Go to Settings > Messages and turn MMS Messaging on.
- Go to Settings > Cellular and turn Cellular Data on.
- Go to Settings > Cellular and turn Data Roaming on if you're roaming on a cellular provider network that is different from your billing provider's network.
- Make sure that you have a cellular data connection in the status bar of your iPhone.
- Go to Settings and turn Wi-Fi off. Open Safari and navigate towww.apple.com to verify that you have a data connection. Turn Wi-Fi back on to continue using Wi-Fi for other features. If your cellular data connection isn't available, follow these steps.
- Make sure that you can send and receive SMS. If you can't send and receive SMS, see the "Issues with sending and receiving SMS" section, above. MMS might not be available while you're on a call. Only 3G and faster GSM networks support simultaneous data and voice calls. Learn more about which network your phone supports.
- Restart your iPhone.
- Tap Settings > General > Reset > Reset Network Settings on your iPhone.
- Remove your SIM card and insert it again.
I hope this helps,