Yes, there needs to be a fallback method where the OTP will be sent to the phone number as well.
WhatsApp might give a response that the phone number doesn't exists on WhatsApp and we can try to send the OTP via SMS, but what if the number is registered on WhatsApp but is not active on the customers phone.
My suggestion will be to not to use this for OTPs.
And only use this for Notification/Marketing purpose. (For customer, Store owners, delivery guys)
But all upto your business.
Will think and discuss further on this when we start working on this. (This is on the roadmap, but the timeline is not yet decided for this feature)