Morse code for SOS, the now-obsolete international distress signal. Far too close to Morse for SMS, which is ... -- ... and lots of mobile phones are set to beep this code when a new text message arrives. This always makes me look to see who is in trouble.
I think they should beep -.-- --- ..- / .... .- ...- . / .- / -. . .-- / - . -..- - / -- . ... ... .- --. . instead.