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Byte Magazine, V18, N8, July 1993, pg 184 has a good background article about
escape sequences.  The information below is a less technical explanation of 
escape sequences.

An escape sequence switches a modem from transmission mode to command mode.

Sometimes, an AT command needs to be issued to the modem when it is on-line 
and connected with another modem.  Since the modem is on-line, typing an AT 
command would send the AT command down the connection to the other modem.
Thus the local modem never receives and acts on the AT command.  An escape 
sequence is needed to bring the local modem into command mode (without dropping 
the connection to the other modem).

One escape sequence is to drop the DTR (Data Terminal Ready) signal on one of 
the wires in the serial cable.  This is a reliable escape sequence.   Some 
hardware platforms do not have a wire for the DTR signal and therefore cannot 
perform this escape sequence.  Another type of escape sequence is needed.

An alternate escape sequence is a pause, followed by three escape characters, 
and then another pause.  This escape sequence then puts the modem into command 
mode, allowing entry of AT commands.  (The pauses prevent the modem from 
mistaking escape characters in the data stream for "true" escape characters 
in an escape sequence.)

Hayes has a patent on the pause, escape characters, and pause technique.
Other modem manufacturers are required to pay royalties to Hayes for use of
its patent.  Some modem makers are not using the Hayes patent or any other 
method of distinguishing real escape characters.  This causes factory 
configured modems from these modem manufacturers to inadvertently go into 
command mode when the Hayes test file is transmitted.

Taken from Byte Magazine, V18, N8, July 1993, pg 184 without permission:
"Zyxel [sic] has its own algorithm, for which it claims compatibility with 
existing code.  Since the Zyxel [sic] algorithm is proprietary, we can't 
comment on its strength or weakness.  However, it caused no problem in our 
testing."

Taken from BoardWatch Magazine, V6, N9, November 1992 without permission:
"To illustrate the technical elegance of this [ZyXEL] modem, recall our article 
on the Hayes brouhaha over their fixed guard time escape sequence under the
Heatherington 302 patent. Hayes has licensed numerous modem manufacturers to
use this escape sequence.  A few have not licensed it and often, their modems
will escape to command mode while transmitting files containing +++ escape
sequences.  Hayes caused something of a furor in July by releasing a text
file that if transmitted by many modems that don't use the guard time escape
sequence technique, would abort the transfer and improperly escape to command
mode. Multitech's  modems fail the test rather awkwardly.  The ZyXEL modem
does NOT license the Hayes escape sequence.  According to Gordon Yang, they
use a proprietary variable sampling algorithm that does the job at least as
well.  We tried the ZyXEL on the Hayes test file - and sure enough, it worked
like a champ. ZyXEL appears to have engineered a way around the escape
sequence controversy.  Yang indicates that they could conceivably publish the
algorithm. If they did, this would take some serious steam out of the Hayes
licensing program."