An AtMega ATtiny 85 chip would be a good choice to monitor an audio level converted into a DC voltage when audio is present. 

I was thinking of using a current transformer in series with a phone wire to produce an AC voltage. This AC voltage thru a series 10 K ohm resistor to the cathode of a 4.3 V, .250 Watt zener diode would limit the high level to +4.3 volts when reverse biased and -.7 volts when forward biased. The other lead would go to common ground. A small value capacitor like .1 ufd in parallel with the zener diode would filter the DC . The internal clip and clamp diodes on the AtMega analog input would provide added protection from over shoot and under shoot. The capacitor would require a bleed resistor, possibly 100 K ohm, to discharge the capacitor.

Another I/O pin would be an output to drive a transistor to switch the relay on to interrupt the power to the cordless phone base station for one minute.

Another I/O pin could monitor the Off hook condition by using an opto isolator to sense the On hook voltage of around 48 volts DC, Off hook line voltage is usually 5 to 8 volts DC.