iPhone (iOS7) keeps sending text messages instead of iMessage suddenly

Hi all,

I'm already searching for quite a while now on this but I can not find a solution.

I had an imessage conversation going on with one of my friends when suddenly one of my messages was sent as a normal text message.

Maybe me or she didn't have an Internet connection at that time, who knows...

But since then all our messages are sent as text messages instead of imessages, even when we are having Internet connection.

I think I already tried about everything now that I could find online:

- Disabled "Send as SMS"

- Disabled imessage and facetime and reenabled them

- Reset network settings

- Reboot

- Removed the conversation

I'm getting pretty frustrated now with this.

So anyone of you has any other suggestions?

Thanks!