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.
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: