This is a basic DAA that I have built in the past. Simulate this circuit – Schematic created using CircuitLab The actual circuitry of a DAA is not difficult: a 1:1 ratio 50V transformer, optoisolator for ring detector, reed relay for a hookswitch and a capacitor to couple audio will make a usable one. You can probably find enough parts online for a one off, otherwise, Microchip has published code to make a PIC decode DTMF.
Texas Instruments also used to make a few DTMF decoder chips. To build your embedded controller, you would then add a DTMF to binary decoder such as the now-defunct Silicon Systems SSI202. It comprises the isolation from the telephone line, audio balancing hardware, hookswitch, and typically a ring indicator for incoming calls. If you can't find one that is designed to detect tones, then what you need is the part of a modem called a Data Access Arrangement. Useful for a reality check on one-way audio or excessive audio delay.An ordinary modem will not be sufficient if you want to pass DTMF.
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