I am incredibly annoyed with Apple about this same issue. I also switched to a Nexus 5. I called the number you listed, and they confirmed that iMessage is alr

I am incredibly annoyed with Apple about this same issue. I also switched to a Nexus 5. I called the number you listed, and they confirmed that iMessage is already turned off for that number (which I did on my iPhone after switching to a Nexus 5 as well). Most of my friends' iPhones will still send iMessages to me, and most of the time their phones don't even say delivery of the messages failed. The tech support guy at 1-800-MY-APPLE told me that I should ask all of my friends to remove their existing text message threads with me from the messaging app. As the only proposed solution to my problem, this is already annoying enough. To make matters even worse, I just had one of my friends do exactly what they suggested, and his phone is STILL trying to send me iMessages. It's unbelievable that Apple shows this little regard for their former customers. (Not to mention the fact that as an owner of several Apple products, I'm actually a current customer.)