Having the same issue. It seems to be an issue with imessage and not the carrier or your phone itself. I stopped receiving group messages from people who were still on iphones, they could see mine but i would not get theirs (i just switched from an iphone4 to the s6 edge). Did the deregistration page thing, turned off imessage on my old phone, but still happening.
I downloaded Textra, and set that as my default messenger, and that seemed to help most of the conversations get through, and is very similar to the way the stock messenger acts. Although the edge notifications for messages dont show up. Hopefully after a month or so I can switch back to the stock messenger and find it to now be working.
DO NOT DOWNLOAD imessage! I think its been pulled from the play store now, but it was a service that allowed you to send messages as imessages, however it was routing the messages through a server in china, likely for data harvesting.
I wish there was a way to route my messages through my old iphone to avoid the imessage issues I am having, or a way to falsely identify your phone as an iphone to continue the messages going through lol (basically copy whatever the china servers were doing without having to use them).
oh well... isn't apple wonderful...