Good morning RogerH71,
Thank you for sharing your concern with the community. As your wireless service provider, we want to make sure that you are able to use your device to its maximum potential! I'm certain that we can partner together and get this issue resolved for you once and for all! May I ask what software version do you have on the iPhone 4S? Can I assume that you are using the default dialer on the iPhone?
Since there isn't a setting in the device to adjust this feature, an alternative solution would be to download a DTMF dialer apps in the App store. If you prefer not to use an app for this function then the next step would be a hard reset. As always, I recommend backing up all of your personal information before performing this reset. Here's a link on how to reset the iPhone 4S.
Once you have completed the reset, please reactivate the device and test the DTMF function before restoring your personal information. If the tones still doesn't work then please advise so we can continue to investigate.
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