In line with the new regulations set by Telecom carriers in the USA and Canada, anyone sending SMS/MMS messages over a 10DLC number (or simply a local phone number) from an application to the US must register for A2P 10DLC.
Carriers consider all SMS traffic from Twilio and other Bitrix24 SMS providers to be sent from an application, so anyone using a 10DLC number with an SMS app to send SMS messages to the US will need to register.
This is a guide on how to register successfully.