WhatsApp for Business does support phone number registration for Singapore numbers provided by Toku.
The number has to be owned by yourself and you are able to receive a voice or SMS during the registration process.
Please review the following on how to use your Toku number to register for WhatsApp for Business.
https://developers.facebook.com/docs/whatsapp/phone-numbers/
We would like to also inform those that are conducting marketing activities towards your attention of the Singapore Do Not Call Register:
Also for those conducting bulk communications using the WhatsApp for Business service we guide you towards legislation regarding the control of Spam: