Hello there dubiness,
Thanks for posting here on the Apple Support Communities! This is a place where other Apple users can submit questions or respond to others to help lend a hand.
We understand you're not able to use the touch tone pad on your iPhone in certain situations. Let's take a closer look as we do our best to provide some additional insight.
To get started, do you see the letters appear under each number with either the white or black background? The reason we ask is as long as you do, it should still function. If not, be sure to test with more than one phone number as not all touch tones may support all devices.
If others work just fine except this one, then you should let them know. If no touch tones work for any other phone numbers, then contact your wireless carrier.
We hope this helps get you pointed in the right direction.
Take care.