I've had an iPhone 5S for 2 years now which has been mostly problem free and may have set a world record for going the longest time without shattering the delicate screen. However, there is a problem I ran into about 2 months ago.

I cant send messages in a particular group chat. I know who everyone is in the group chat, so I know theres not some indian hacker or crazy person involved (at least I hope not). I received someones "old" iPhone 6 a few days ago and after switching to it, the problem is still present. I can receive messages from the group chat but when I reply I get the [MESSAGE SEND FAILURE] notice. I can send messages to all other group chats, just not this one. Most of the other people in the group chat have iPhones, and no one else is having this problem. I called  and they were unable to diagnose the problem. Does anyone have any ideas, or has anyone here experienced this problem?