When you first set up an iPhone or restart it after a factory reset, one of the first things you may want to check is whether Apple's iMessage service is activated. With iMessage, you don't have to rely on your carrier's data plans to send and receive messages, since most of it happens over the internet.
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If you are unable to activate iMessage on your iPhone, you can read the following post to understand why that happens and what you can do to fix the problem.
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The “Activation Failed” error can occur on your iPhone because your device is unable to use iMessage to send and receive iMessages. If you are one of the users experiencing this issue, here are some reasons why it might be happening: