When you register >=1 devices with iMessage (w/ phone number) others who also have iMessage enabled will send iMessages to your device rather than SMS. This is obviously by design. This causes a problem that will affect users without them even knowing so.
To explain the problem I'm going to use the fictional users "Andy" and "Beccy" as example. Assume both users use iMessage:
- Andy is on a data-plan with a limit
- Andy hits his data limit
- Beccy sends a message to Andy
- Beccy receives a "delivered". Giving her assurance that Andy will receive the message
In this situation, unless Andy connects to a WiFi network, Andy is unable to receive the message. Andy would have no idea that Beccy even tried to contact him. Beccy would have no indication he was unable to receive the message