If you send SMS messages in the United States, then action is required.
All other users can ignore this message.
If you are in the USA and use a Twilio account in D4H (Account Owners: Team Settings > Communication) to send your SMS messages, then you likely send from a USA-based 10DLC number.
If you haven't already followed Twilio's prompts to register your number, then you need to urgently register your number to prevent your SMS messages sent from D4H being blocked by Twilio.
Sign-in directly to your Twilio Console to register your number. Twilio are reporting 2-3 week waiting times for registration at present.