I switched to the iPhone 5s a few months back from my android. I have never been able to reply to group texts or picture messages, even though I receive them just fine. This was an inconvenience that I delt with because I wasn't looking forward to hours on the phone.
Until last week when I lost the ability to send all text messages, even though I'm still getting them (Real pain over vacation to not have any messaging).
When I try to send I shows the "not delivered" status of the message.
I have tried turning iMessage off before last week and it didn't help. Also some people receive my texts from my email address instead of my # that they text.