<s>
Guard	B-Device
tone	I-Device
is	O
a	O
feature	O
of	O
wireline	O
modems	B-Application
.	O
</s>
<s>
The	O
guard	B-Device
tone	I-Device
is	O
sent	O
by	O
the	O
answering	O
modem	B-Application
after	O
it	O
has	O
sent	O
the	O
answer	B-Device
tone	I-Device
.	O
</s>
<s>
Guard	B-Device
tones	I-Device
are	O
not	O
used	O
in	O
North	O
America	O
.	O
</s>
<s>
The	O
guard	B-Device
tone	I-Device
is	O
controlled	O
by	O
the	O
AT	B-Device
command	I-Device
of	O
the	O
Hayes	B-Device
command	I-Device
set	I-Device
.	O
</s>
<s>
North	O
American	O
Bell	O
103-style	O
modems	B-Application
(	O
300	O
bit/s	O
)	O
were	O
incompatible	O
with	O
UK	O
in-band	O
signalling	O
(	O
the	O
modem	B-Application
tones	O
would	O
disconnect	O
the	O
call	O
)	O
unless	O
a	O
guard	B-Device
tone	I-Device
is	O
provided	O
.	O
</s>
<s>
Most	O
of	O
these	O
(	O
now-obsolete	O
)	O
modems	B-Application
did	O
not	O
support	O
any	O
command	O
set	O
and	O
were	O
unusable	O
for	O
international	O
calls	O
.	O
</s>
