If you have a natural ear for music and have perfect pitch, then tuning any sort of instrument, including the guitar, is probably not a problem for you. But for the rest of us, a piano or a pitch finder is necessary to make sure our guitars are in tune.
But what if you're traveling and have neither with you? Check out this video to see how to tune your guitar using the dial tone on a U.S. phone.
The U.S. dial tone is 440 Hertz that matches the pitch of the open 5th (A) string for standard tuning on a guitar.