Once you start using Sirena with the WhatsApp Business API, you cannot use that number without another API provider. This is because WhatsApp's user policies do not allow a WhatsApp Business API account to be switched to a basic Business or personal account.
However, you still have some options that will allow you to use the number:
Use the number with Sirena, with the WhatsApp Business account you already created.
Migrate the WhatsApp Business account you created in Sirena to another API provider. Sirena is not involved in this process. You'll have to sign up with another provider who works with portability and request it.
Delete the phone number for the WhatsApp Business account and delete the account itself and wait about 90 days. The days start counting from the day the number is de-registered on Facebook, not necessarily from when you delete your account in Sirena. Either way, you can go to your Facebook Business Manager and delete it yourself.
If you do not complete the process of integrating the WhatsApp Business API with Sirena, or if you delete your account before finalizing the process, your account will continue as it is.
In summary, here's how moving your number between the various types of WhatsApp account works:
WhatsApp personal to WhatsApp Business API. ✅
WhatsApp Business to WhatsApp Business API. ✅
WhatsApp Business API to WhatsApp Business API with another provider. ✅
WhatsApp Business API to WhatsApp Business. ❌
WhatsApp Business API to WhatsApp personal. ❌