I sympathize with your problems...I recently switched from an iPhone 3gs to an android phone on the same number. This led to messaging problems to and from my new android to iPhones in the family (not to mention I assume many other iPhones). So I talked to 6 different ATT retail employees, several chats with both ATT and Apple, and did lot of reading here and elsewhere.
Here's my take, for what its worth...an iMessaging only iPhone cannot send messages to an android phone. If MMS/SMS is ennabled on an i'Phone also having iMessaging ennabled, messages may (or may not) reach an android phone depending on whether or not a particular iPhone in the family (or elsewhere) has been used already to message the person who used to have an iPhone but now uses an android on his or her same number. In such a case we are being told that the iPhone user must delete the thread of previous messages to the new android owner's number in addition to my unregistering the old iPhone, turning off iMessaging in the old phone before activating the new android on the same number, or other possible actions....
The point, really, is that iMessaging and SMS are incompatible systems. In doing what I did, switch from an iPhone to an android on the same number, I disconnected myself from some iMessaging only users without realizing it. Finding this out after the fact (and I consider myself to be a well-read person having read most of the 264 page android user manual) made me upset because I was then in the position of having to fix this problem iPhone by iPhone, number by number, despite assurances from Apple support that all would be well after they deleted my number from the iMessaging system and/or Apple ID. To this point two of my iPhone using family members cannot text me because they haven't taken the steps necessary on their phones to make it happen. Their messages to me are listed as "undeliverable". And so it goes.
It would be nice of course if an iMessaging only phone could reach an SMS messaging phone without user intervention, making a switch from one messaging system to another. That, I would guess, will happen no time soon.
Best wishes, Kent