I used to pulse dial a phone using the hook switch to get free calls. There was a phone in the bar in my college that only accepted incoming calls. It did this by having no dial. Of course, that didn't stop you from tapping out the phone number if you could get the beat right.
The other stupid phone that I hacked around was a pay phone in a flat that I rented where you'd pick up the handset and get a dial tone but the keypad was inoperative unless you put in money. Hello DTMF pocket dialer.
It's not unreasonable to think that someone could emulate the carrier tone with their voice. After all the modem had to work inside the frequency range used for voice communications.
The tones used in DTMF were chosen to minimize accidental talk-off. They're relatively widely separated, with no harmonic relationship between any given pair.
The other stupid phone that I hacked around was a pay phone in a flat that I rented where you'd pick up the handset and get a dial tone but the keypad was inoperative unless you put in money. Hello DTMF pocket dialer.
It's not unreasonable to think that someone could emulate the carrier tone with their voice. After all the modem had to work inside the frequency range used for voice communications.