"Precise" dial tone (in North America) is a linear mix of 350 and 440 Hz, so that can be used as a reference. France usually uses pure 440 Hz, and many countri

"Precise" dial tone (in North America) is a linear mix of 350 and 440 Hz, so that can be used as a reference. France usually uses pure 440 Hz, and many countries use 425 Hz. Of course, with cell phones, a lot of people don't even have a clue what dial tone is. :)