How do I fix 'Unknown Error' when activating iMessage and FaceTime?

Well, there must simply be credit in the SIM if it is a SIM card with a number, but the email does not need a fee for that, because activating both applications requires a fee on the SIM card, and also you may need a Wi-Fi connection, that's all and thank you.

Check your network and make sure your date/time settings are correct. If that doesn't help, try signing out of your Apple ID, restarting your phone, and then signing back into iMessage and FaceTime.