Nortel Meridian SL-100 Reference Manual page 233

when the escape sequence +++ is entered while the MCA is on-line
when a call is dropped using a valid break sequence
Procedure 9-1
Hayes keyboard dialing escape code sequence
1 Wait at least one second after previous entry.
2 Type +++. Space each keystroke less than one second apart.
3 Wait at least one second before the next entry.
Prompt:
The following message is displayed:
OK
The MCA returns to command state.
4 You have completed this procedure.
In the Hayes command state, your input is interpreted as call setup
information. When the call is established (indicated by the CONNECT
prompt), the MCA enters the online state and transmits all data on the
RS-232-C interface transparently across the network to the far-end MCA.
The MCA attempts to go online in the following situations:
when the dial command D is issued
when the online command O0 or O1 is issued
when the answer command A is issued
when the ring count (value of register S1) equals the numbers of rings to
answer on (value of register S0)
In this state, the MCA acts as a full duplex device, and it is the responsibility
of the DTE connected to the far-end MCA to respond to your input.
The return from online state to the command state may be performed by
using an escape code sequence. The default escape sequence consists of a
minimum of one second of no data followed by three plus signs +++ with
less than one second between each adjacent +, followed by a minimum of
one second of no data. It is extremely unlikely that such a sequence would
be executed accidentally while in the online state.
To return to the command state while also dropping the call, execute an
escape sequence of three "breaks". This is not a conventional Hayes
command sequence, but proves to be very useful. Both the escape guard
time (one second) and escape character + are programmable using Registers
Meridian SuperNode MSL-100 Meridian Modular Telephones Reference Manual MSL11
Hayes keyboard dialing 9-9