Farallon Communications 612 - Manual (Page 117)

Modem Operation A-7
Hanging up
To disconnect the modem from the phone line, type +++ to enter
command mode, then type ATH and press the Return key.
Clearing the command buffer
The command buffer acts as memor y, storing up to 40 command
characters in a single command line. If you get an ERROR response,
tr y typing AT to clear the buffer. The ISDN modem should return the
OK response indicating that it is ready for new commands.