How do I delete or change my number in my old iPhone, so that I can still use it for iMessages without issue?

I have an iPhone 4S that was with Verizon. About a week ago I ported into Virgin Mobile and bought a new phone to use until the next iPhone comes out. I now still have my old iPhone, which I want to use for iMessages, as well as like an iPod, etc.

The issue I am seeing is that my phone number is still registered to my iPhone and I haven't yet figured out how to delete it or change it. The problem this is causing is it is making my iPhone sometimes intercept messages from those with iMessage who are texting my phone number. The messages should be going to my new phone, not my iPhone, since they are texting my phone number (not my iMessage email) and my iPhone has no service connected to it.

Here's an example of what I'm trying to say: the other day I texted a guy selling an item online. I used my new phone to text him, not my iPhone. He texted me right back, but the text went to my iPhone, not my new phone. We then texted each other via iMessage and no texts were making it to my new phone.

I think this is also causing some messages to not go through on my new phone either, where I'm just simply not getting texts from people sending me messages. My iPhone isn't getting the messages, nor is my new phone.

What I need to do is delete my number or put a random number in my phone so that I don't keep missing or losing texts. I saw the ways recommended to disconnect your number from iMessage, but that then kills my ability to use iMessage on my iPhone. There must be a way that the number in my iPhone can be changed or deleted. We are in 2013 I think, and this was something I always had the option of changing via manual programming on every phone I've had since 1902 or so.

How to do this?!