I gave my husband my old iphone and now my texts and emails go to my phone and his?

I don't even know what this is called so googling the answer is impossible. I've tried 'pairing' and 'linking' but to no avail. Do you know what it's called and how I can stop it please?

Added (1). Just to be clear, I bought myself a new iphone 4S and his is a 3GS

Added (2). And this is a new problem, they were working independently fine until 24 hours ago. I have to say my husband isn't the most techy minded (says me!) and would have just pressed ok to anything that came up on the screen.

Erase all the memory, iphones have in built memory