How to stop texts from going through an iphone when you get a different phone?

I just switched from an iPhone to an HTC one. Now when I text my friends the replies show up on my old iPhone do you know how I can stop it? I know that it has something to do with imessage being over Wi-Fi so my phone works w/o a sim.

Do the replies show up on the HTC phone? If so check the Iphone for a default setting, also called 'factory setting'. Also just take the battery out.