<s>
A	O
trunking	B-Protocol
gateway	I-Protocol
,	O
also	O
known	O
as	O
a	O
PSTN	O
gateway	O
,	O
is	O
an	O
interface	O
between	O
Voice	B-Application
over	I-Application
Internet	I-Application
Protocol	I-Application
(	O
VoIP	B-Application
)	O
and	O
Public	O
switched	O
telephone	O
network	O
(	O
PSTN	O
)	O
.	O
</s>
<s>
The	O
gateway	O
connects	O
the	O
VoIP	B-Application
line	O
and	O
PSTN	O
line	O
so	O
that	O
an	O
end	O
user	O
can	O
use	O
PSTN	O
phones	O
to	O
make	O
a	O
call	O
over	O
the	O
Internet	O
with	O
VoIP	B-Application
.	O
</s>
<s>
Trunking	B-Protocol
gateways	I-Protocol
are	O
generally	O
larger	O
than	O
residential	B-Application
gateways	I-Application
,	O
managing	O
a	O
large	O
number	O
of	O
digital	O
circuits	O
.	O
</s>
<s>
The	O
name	O
"	O
trunking	B-Protocol
gateway	I-Protocol
"	O
refers	O
to	O
the	O
Class-4	B-Device
telephone	I-Device
switches	I-Device
these	O
gateways	O
often	O
connect	O
to	O
,	O
which	O
are	O
also	O
referred	O
to	O
as	O
"	O
trunking	O
switches	O
"	O
because	O
of	O
their	O
use	O
of	O
trunks	O
.	O
</s>
<s>
A	O
trunking	B-Protocol
gateway	I-Protocol
might	O
connect	O
to	O
,	O
for	O
example	O
,	O
T1/E1	O
and	O
T3/E3	O
circuits	O
.	O
</s>
