A girlfriend to one of my friends recently decided to leave the dark side and replace her iPhone with an Android (good decision!). However when she was using the iPhone she also had enabled the iMessage texting.
After switching to her new Android phone her friends, that still was using iPhones, tried to text her. Because her iMessage account was still enabled the texts ended up in the Apple void and didn’t reach her. To disable it the SIM-card had to be put into an old iPhone 3 in order to turn the iMessage off. The iPhone she replaced was using micro-SIM but the Android had normal SIM:s so she had a new larger SIM that wouldn’t fit in her old iPhone. The iPhone 3 works with normal SIM cards so that is why a iPhone 3 had to be used to disable it.
So, if you decide to convert from iPhone to another brand, remember to turn iMessage off in the iPhone first!