In some circumstances, you can move an existing WhatsApp number to Twilio.
However, if you signed up for WhatsApp using the WhatsApp Business API outside of Twilio (either directly with WhatsApp, or through a different Business Solution Provider), you cannot move that existing WhatsApp Business number to the Twilio platform. This is due to current limitations on moving numbers or transferring ownership of existing business accounts.
Moving an existing WhatsApp number to Twilio to use for WhatsApp
If your number was activated for WhatsApp using one of the WhatsApp mobile apps, and not the Business API, you may be able to use it on Twilio. You will first need to host or port the phone number to Twilio, and then go through the standard WhatsApp onboarding process.
Please note, both of these options involve moving the number's SMS and/or Voice capabilities to Twilio. If these options do not work for you, we recommend buying a Twilio phone number to activate for WhatsApp. Many types of Twilio phone numbers can be enabled for WhatsApp messaging, even if they are not SMS-capable. For more info, see Which Twilio Phone Numbers are Compatible with WhatsApp?