Okay, I had to sign up just so I can post because I am so sick of all the misinformation that floods these comments sections when it comes to the infamous "imessage problem". You can accept this or ignore it, I don't care but my information is right.
1. When you activate imessage it registers your phone number for the service
2. This is how other imessage users, ios or osx device, reach you using the service
3. When you deregister your number, it does remove your number from imessage service within 24-48 hours
4. The problem usually lies in either
A. your contact is still sending message to you as an imessage (typical in cases seen in group messages aka "this happens X amount of years after i've switched"
B. even though they have deleted your contact and readded it multiple times the message thread they still have going with you is an imessage thread and needs to be deleted as well
If you, friend, or family are having this issue still all these things must be done for each and every contact that still is using ios and osx. As long as your have a device with an active thread that still thinks its using imessage i will continue to try and use imessage instead of defaulting to SMS. Of course, you cant always tap and hold and choose send as SMS instead.