Thought for the day…
A standard dial tone is made up of two simultaneous frequencies: 350Hz and 440Hz. As any musician will tell you, 440Hz is the A above middle C – the concert pitch standard adopted in the 1930s. 350Hz is the F just below it.
Some modern phones (such as my DECT cordless handsets) generate their own dialtone, which for some reason is at a different pitch, and mobiles, of course, don’t have one at all, but if you lift the receiver on a standard landline, you should be able to recognise the two notes.
So if you find yourself needing to tune an instrument, or set the pitch for a choir, but you don’t have a tuning fork handy, just turn to the nearest telephone!