This command sequence:

 +++ATH0

1. +++ (three plusses) This character string stops the modem shoveling characters back and forth, and still keep the connection, while trying to interpret what´s coming from your side. Manufacturers call this state "command mode".
2. AT (A and T) Asking the modem for attention. Another command might be coming.
3. H0 (H and zero) The modem is asked to hang up.
4. <CR> (CarriageReturn) This is the same as return on your keyboard, to get the previous commands performed.

  The story behind this is the following:
When Hayes originally designed the AT-commands for modems, they thought of a way to break the current work but still keep the connection. It was called the Hayes Improved Escape Sequence with Guard Time (U.S. Patent #4,549,302, granted 1985). It consists of two parts.

1. The Guard Time that precedes and follows the Escape Sequence so that no accidental interruption might occur.
2. The Escape Sequence is three equal characters in sequence.

  To be able to modify the value of these, they are hosted in registers on the modem. Register S2 for the escape character(default = 43) and S12 for the guard time(default = 50).

  Arranging for a low to nothing price on the modem, as a producer, you might skip the payment for the patent. And as a consequence, skip the implementation of the Guard Time timer. I don´t know if, by not implementing the Guard Time, you may circumvent the patent royalties.

  Of course, it´ll be tough to use an alternative implementation. Not many communication software companies want to make a zillion versions, to test and market.

  But if nobody tells the customer about it, you´ll never know. And you´ve learned that an occasional broken connection might happen. You reconnect and that´s it.

  However, it might be another reason...