Can't deactivate iMessage after switching phones?

I bought an Android phone tonight, replacing my iPhone. I also ported my number to the new phone. However, now, anytime someone with an iPhone tries to SMS me, their phone automatically sends it as an iMessage, which I can't receive, since I no longer use my iPhone. How do I deactivate my phone number with iMessage so that I can start receiving text messages from my iOS-using friends again?