This device listens for dial tones from any mobile phone via a microphone.
So it can be concealed behind the door or something.
No special apps are required on the phone - just the built-in dialer tones.
When the correct code sequence is received, the lock pin goes high and unlocks the locker.
If the code sequence is the master code, you can change the unlocking code.
Both codes are 6 digits. Standard 0 to 9 * and # tones can be used without any app.
Better use with MAX4466 amp.

More details and sources on github.
https://github.com/Airrr17/DTMF_locker