Hi, I switched from and iphone to an android phone just over a week ago. I had the same issue with messages being blocked to my android phone because of imessage.
My carrier advised, in my iphone, to go to settings - phone and delete your mobile number in the "my number" area then type in a dummy mobile number (e.g. 0400 000 000) and save. This helps but did not fix the issue.
My dad and I worked through it last night as he has an iphone. You need to break the link between imessage (and possibly icloud) and your mobile number. I believe the solution is as follows:
1. Do as my carrier advised (above).
2. On iphone - go to settings - messages and turn off imessage, and in send & receive (below imessage option) untick your mobile number. In settings - face time, turn off face time. If you have an icloud account delete that as this may be retaining a link to your phone number as well.
3. In Messages on your iphone - clear any converations you have with anyone who has an iphone and delete the "folder" attached to the conversation.
4. Now, if you have any other Apples devices which have been linked/synced to your mobile number you will need to go into Messages and clear all conversations and delete the associated folders. In settings - messages - send & receive untick your mobile number, this will most likely just leave you with an email address.
5. The link may need to be broken similarly on friends who have iphones for your mobile number only - i.e.clear conversation history in messages.
6. Most importantly if this doesn't already change your contact from blue to black following a reboot of their iphone - i.e. change blue imessage capability to green text message cabability, you should request that they change the label of you phone number in their contacts from "iphone" to mobile. Alternatively if friends have an iphone 5 i believe they can change your number to a non - iphone contact.
Hope this helps
I don't know why Apple is allowed to get away with this message blocking through syncing/linking imessage!! ... perhaps a ploy for Apple domination... Another reason why I changed to Android!