Uncouple, De-register Your Phone Number From Apple's iMessage System.

De-register from iMessage system

Up until now, people who switched to another phone system like Android from iPhone, had their text messages disappear  in to ether. These users did not have an easier way to uncouple their phones from iMessage system. That was because the phone number was still tied to the Apple's iMessage system and text from Apple devices to the said number were routed to the iMessage. Frustrated users even filled a class action suit against Apple for this issue. Perhaps Apple is paving a path to face the law suit.

So if you are planing to switch from an iPhone to another phone, the best route is to disable iMessage, BEFORE switching from the iPhone. (If you still have the phone, you can reinsert the SIM and accomplish the task as well)

But as one might to do this in the rush to switch to the new phone, be it an Android phone or Windows phone and have already disposed the iPhone, Apple has provided a webtool  de-register your phone number from the #iMessage system.

You go to the webpage, select your country, enter you phone number and after receiving a text message from Apple with a code, you enter the code into the same tool. You are uncoupled from the iMessage system.
Apple via Reddit   MobilePhoneNewz