Can't compose or send text with siri using iPhone 4s with iOS7. Siri will do other tasks like call, reminders, etc, but when I ask, "Send text to _______" it thinks for a while and then says something like, "I'm really sorry about this, but I can't take any request right now."
My spoken request appears in text form in the upper right hand of the screen so I know it has translated my voice to text, but for some reason, it won't complete the text option.
I was able to text using Siri earlier, however I switched to Samsung Galazy S4 in September. Didn't like it and just switched back to my old iPhone4s. That's when the problem was first noticed.
Maybe a soft reset will fix Siri. You don't have to wipe your phone or connect to a computer to do this. Just hold down the home and lock buttons for ten seconds. Ignore the Slide to turn off banner. Then turn your iPhone back on normally by holding the lock button. This often fixes glitches like this.