Pin15 goes high if a valid DTMF-tone combination has been received. RN1 is a four resistor array with a common ground pin meant as pull down resistor for the outputs Q1 through Q4 (A0 .. A3). Q1 .. Q4 are binary coded outputs with Q1 being the LSB.
Please download the data sheet and study it carefully. It also contains a look up table for binary outputs according to frequencies generated by DTMF devices.
Other than binary values the CM8870 decodes a 'Zero' being "0000", instead of "1010", provided by a binary coded switch.
So if you want to create a DTMF-lock don't use 'Zero'.
Boncuk