...
AST_MIN_DTMF_GAP: Minimum amount of time between the end of the last digit and the beginning of a new one. This is defined in main/channel.c. Change this value to 35.
AST_MIN_DTMF_DURATION: Minimum allowed DTMF digit length. Defines in main/asterisk.c. Change this value to 45. (Note: "mindtmfduration" configuration option might present "asterisk.conf" in latest asterisk version, so we can change the same from configuration file as well).
After changing above two fields, perform clean compilation of asterisk i.e. "make clean && make && make install".
After recompilation, again perform "configuration" section steps.