Ericsson ZAT 19.2 Operation Manual page 42

3.3 +++ Escape -Sequence
Command: +++
When connection is established with the remote modem, an AT
command is ignored by the modem. +++ causes the modem
returning to command state without going «on-hook».
The modem returns to command state
The modem responds with resultcode: OK.
"The +++ escape sequence is protected by guaratimes to
inhibit return to command mode inadvertently. |e. there
must be no data too close in time before the first +, not
too long time between the +, and no data too close after
the last +. This time is set by the S12 register (see
chapter 4).
3-4 LZTR104006 R1.00
Zat 14.4/14.4-7 and Zat 19.2/19.2-7