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I don't even know if this will work, but after having this problem intermittently on an ATT galaxy note 4 and not being able to find DTMF settings ANYWHERE, I found this:
Settings>
Device>
Personalization>
Accessibility>
Dexterity and Interaction>
Press and hold delay (options short, med, long)....
On a T-Mobile galaxy note 4 it was similar but not exact:
...
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