I've looked in Settings and can not find anything that might lead me to the DTMF adjustment from short to long for my hearing aid app.
Open the dialer, press the 3 dots in the upper right corner (in the search bar), select Settings, Calls. Hearing aid comparability and DTMF settings are there.
I do have the US model. The setting for DTMF is right below the setting for hearing aid compatibility. The choices are Normal and Long.
Maybe it's carrier specific? I'm on Verizon if that makes a difference.