How to Enable DTMF on Nexus 5 running Android 6 Marshmallow

Unlike the recent phones, there is no separate setting to Enable/Disable DTMF in Nexus 5.
However, if you are facing issue with touch tone dialing, you can find the setting to change  under Settings->Accessibility-> "Touch and hold delay". Change to Medium or Long. This fixes the touch tone/dialpad not working during a call. At least it did for me. A small downside is you will need to press the keys longer, but it will work!