When I was a kid, I saw a toy phone that would normally play DTMF tones when you pressed a key, but also had hidden sounds for different key combinations. This

When I was a kid, I saw a toy phone that would normally play DTMF tones when you pressed a key, but also had hidden sounds for different key combinations. This was at a family gathering, so I was naturally very bored and brute-forced them all, at least the 2-key ones. I don't remember what all the hidden sounds were, just different ringtones I think. Now the engineer within me wonders if the DTMF tones it played were actually accurate.