Only works if the caller can be accesses with the same ID on both devices. If someone sends you a iMessage to your phone's number, it will not show up on the iPad. If they use your email address, and both devices are set up to receive using your email address, it should work.
Hold in mind the iPad will not work with SMS, period. So if you get a SMS message on your iPhone, do not expect them to show up on the iPad. Only messages between iOS devices will work on the iPad.
I have a lot of trouble testing this, since I only have myself to send messages to and from. That really confuses things. I get everything on both devices but sometimes I end up splitting the conversation into two streams.