How to add new number to iMessage?

Ok, my friend gave me his old iPhone, which I use as an iPod. There is no SIM card, but I was wondering if there was anyway to change the number (his old number) on iMessage into my phone number?

Under Send & Receive, I am able to list my own email, my Apple ID, however, I am unable to tap on his number, it's gray.
Any suggestions? Or will I not be able to use iMessage if I don't have a SIM card?

Also, under the button to turn iMessage on and off, it says "waiting for activation". This is probably why I can't change the number, but where is it waiting to be activated? Where can I go to activate it?

Hope this makes sense..
Thanks!