dtmf tones not working on my iPhone 4

If I am ill I need to call in to my school district to report an absence. This procedure has worked fine with my iPhone until about 4 weeks ago. Now when I call in the system says that the number entered is incorrect, even though it worked before. My school district tells me that the dtmf tones have to be enabled and set to long. I have no idea what that means or how to do it. I would appreciate any help. Thank you.