Update May 2021: Twilio is no longer offering China SMS template registration. Changing regulations and heavy message blocking on seemingly legitimate registered traffic have made it impossible for Twilio to guarantee a high-quality customer experience for SMS messages to China mobile users.
After June 20, 2021 at 6 PM Pacific US time, all SMS message traffic to China sent via Twilio will have limited support. This means that messages may deliver with a generic signature prepended to the message, and messages may deliver with arbitrary local phone numbers as the Sender ID. Twilio will not be able to assist with undelivered messages to China.
If you send one-time passwords (OTP) or authentication codes, Twilio Verify remains an option for sending this type of SMS to users around the world, including in China.