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Check your device settings
Make sure that you’re connected to a cellular data or Wi-Fi network. If you're using an iPhone, you need SMS
messaging to activate your phone number with iMessage and FaceTime.
Depending on your carrier, you might be charged for this SMS.
Go to Settings > General > Date & Time and make sure that your time zone is set correctly.

Turn off and restart iMessage and FaceTime
Go to Settings > Messages and turn off iMessage.
Go to Settings > FaceTime and turn off FaceTime.
Restart your device.
Turn iMessage and FaceTime back on.

Still not activated after a day? It might take up to 24 hours to activate iMessage and FaceTime. If the issue continues after 24 hours,
follow these steps:
Make sure that your device has the latest version of iOS.
If you're using an iPhone, contact your carrier to make sure that you
can receive SMS messages.
Contact Apple Support.
Ha i link per supportare una volta che hai escluso le cose semplici come un'impostazione sul tuo Mac. Possono aiutarti se la tua connessione di rete non raggiunge i server di Apple o se c'è un problema con il tuo account sul lato server.