Yes, you can use an existing WhatsApp number with Botsup. However, you will need to first delete the account linked to that, prior to onboarding with Botsup.
If number is being used by another Business API, you still can get it migrated to Botsup after getting the 2 Factor Authorization (2FA) pin disabled from your current BSP. Or, you can just provide a new phone number on which you can receive an OTP - this could be a virtual number or a regular SIM number.
Once migrated to WhatsApp Business API and linked to Botsup, the number can be accessed on Botsup only.If it is a virtual number that is tied to Botsup, then normal voice calls can be made as usual, but WhatsApp voice and video calls cannot be supported via WhatsApp Business API.