This command to a modem:

 +++ATH0

  Simply reading this text, or any other modem command, should never affect the modem. But a cheap design in the modem´s command processor can allow this to happen.

  It´ll open up a security hole and also knock you off-line at unexpected times.

  If nothing happened now, your modem is secure against incoming command sequences.

  Btw, where do you encounter this or similar character sequence? In a mail sent to a friend, telling about something new you´ve been reading :-) There might be in a uuencoded file and in EDIfact files, where you have plenty of them.