A lot of people have trouble activating their new iPhones. If you’re getting a message about the activation server being unavailable, it could be that Apple is simply struggling to cope with the current volume or people trying to access it. You can check the status here — if there isn’t a green box next to iOS Device Activation, then try again later.
1, If you’re trying to do it using your cellular connection, then switch to Wi-Fi instead.
2, Try turning your iPhone off and on again by pressing and holding the Sleep/Wake button and then dragging the slider to turn the device completely off. Press and hold the Sleep/Wake button again to turn it on.
3, Make sure that iTunes is up to date, and try plugging your iPhone 7 into your computer and completing the activation using iTunes.
4, Contact your carrier and ask if there are any known issues with iPhone 7 activation. You might need to get a replacement SIM card.
5, If nothing has worked, then it’s time to contact Apple.