For you do-it-yourselfers, standard busy signal (for called number busy, not network error) in North America is a combination of two tones, 480Hz and 620Hz, with a cadence of 0.5 seconds on, 0.5 seconds off. A network error (also known as a fast busy signal) uses the same tones but with a cadence of 0.2 seconds on, 0.3 seconds off. (This is defined in CCITT Recommendation Q.9.)