Its not that simple. WhatsApp only works on your smartphone, your number only works with one WhatsApp account. When someone sends you a WhatsApp message while your data is disabled, the sender can see that the message didnt go through. SMS is then a fallback solution.
iMessage has no delivery indication, only a read receipt (which is optional). iMessage prevents iPhone users from sending SMS in several cases, because there is no granular control over iMessage. Assuming the scenario above, that the iPhone has no data connection, one of following outcomes is possible:
- Messages can send iMessages normally, but the messages are not delivered to the iPhone
- Messages can (re)send any iMessage as SMS manually (by holding a finger on the sent message)
- Messages automatically sends an SMS when the iMessage cannot be delivered to the recipients iPhone
- Messages defaults to SMS for that contact
- Messages cannot send either iMessage and SMS anymore
In practice, anything could happen in that case, depending on the service itself and the user settings.