Problems using Siri on IPhone 4S?

I just bought the IPhone 4S a week ago. I purchased it mainly for Siri. She works great and is really helpful, unless I'm speaking to her through my mic on my headphones. (I use my headphones while I'm at work since we're not allowed to have our phones out. It's easier to control my music this way. I'm using the headphones that Apple provided with my phone.) She hears me just fine when I tell her to turn on my music or read the new text message. But if I tell her to text someone or call someone she can't understand me. I'll have to repeat the command 5 or 6 times and talk slowly to make her hear me. And when she finally hears that I want to send the text to someone, getting her to understand what I want it to say is almost impossible. The thing I really don't understand is she understands me just fine in my car when I have her plugged in to my stereo. Anyone else have this problem or know what I'm doing wrong?