Many people may know that the DTMF tones (the “beeps” you hear when you punch the buttons) are copyrighted by the phone company. What you may not know is that the dial tone, the busy signal, and the off-hook alarm are also copyrighted.
And on an old rotary phone, you could dial a number subtractively. If you wanted to dial a 4 for instance, you could put your finger in the 7 hole and move it to 3. Or 9 to 5. Etc.. The system worked by counting the number of pulses as the dialing wheel passed a sensor, so it didn’t care how those pulses were sent. Which also enabled a user to simply click the hangup buttons a specific number of times in rapid sequence to effect the dialing of a number.
Then there was the 11911 technique. Dial that number and your phone rang back. Never could figure out why. But it was awfully handy when you wanted to call someone to dinner and you knew they were near an extension.
We didn't even have extensions until 1969 or so when my parents bought a rural house from a guy who worked for the phone company.
The dial tone is an “A” and can be uses to tune your guitar.