A:
Roku is an item that is connected to your Television. When using your tablet or smartphone it is connecting directly to your wifi (internet.) Usually, when watching on these devises, you have the browser that can be used to take you directly where you need to go. Also, you can install apps on these devices that you can use to watch TV and movies such as Hulu Plus, Netflix, ect...
From what I understand there is a Roku app available for IPads, tablets, IPhones, and smartphones, but this app is to allow the device to operate as the remote control to the Roku device. However, I think that all the Roku's come with a remote control so I don't know why this app would be needed.
Normally, if your watching something on your IPad, or IPhone when you receive a call, it interrupts what you are watching so that you can take the call.
I hope this helps to answer your question.