The modern dial tone varies between countries, being a "buzz" of two interfering tones (350 Hz and 440 Hz, as defined in the Precise Tone Plan) in the North American Numbering Plan (most of North America), and a constant single tone (425 Hz) in most of Europe. Modems, fax machines, and autodialers must be designed to recognise these so-called call-progress tones, as well as comply with differing standards and regulations.
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http://en.wikipedia.org/wiki/Dial_tone