REG-RESP?

Do you receiving the message when you are trying to activate iMessage and Facetime?

See this Apple article ☞ If you can’t turn on or sign in to iMessage or FaceTime

If you can’t turn on or sign in to iMessage or FaceTime - Apple Support

Perhaps Is your Wi-Fi network Is unavailable?

Turn on Wi-Fi network: Settings ☞ tap on Wi-Fi ☞ turn Wi-Fi ON

You have to make the Wi-Fi network available, because the phone sent 'silent' message at the background to the server, trying to activate your request.

If you are using an unsupported carrier, and you will be charged for the background text messages used to activate FaceTime and iMessage.

It is your choice to activate knowing you may be charged, or to cancel and not use these features.

Or you switch to a supported mobile carrier.