I had the same problem twice in the past. Switching from iPhone 4 to GS3, and then from GS3 to iPhone 4 (after I broke my GS3), back to GS3. It was a nightmare.
As if texting with iphone users (especially in groups!) isn't hard enough. I called apple a ton of times, and I do have to say they were the most helpful. I would call your service provider maybe once, but apple was the way to go for support, for sure. They were very nice and TRIED to help, or so it seemed, but there was not much they could do.
They even had me put my SIM card back in the iPhone, turn off imessage, log out of everything, etc... The issue is that you can't change the imessage settings that include a phone number on the iPhone when it doesn't have a card in it. You'll see the number is grayed out. That was part of the problem for me. But, even after I did everything they said (the second time, especially, which was only 5 months ago), I was still missing texts from iPhone users. However, it slowly progressed to the point where the messages would actually not SEND on their part. So they would be notified "message not sent, send as text message?," which was a huge step up imo. After a few weeks the problem did iron itself out.
Post a message on social media or whatever to tell people not to text you without deleting you and adding you as a contact. Or to tell them to make sure that "the message is green." Gosh, apple users don't even know the difference between iMessage and SMS, just know how to see colors. Hang in there, don't give in!!