<s>
In	O
business	O
telephony	O
,	O
a	O
telephone	B-Application
extension	I-Application
may	O
refer	O
to	O
a	O
phone	O
on	O
an	O
internal	O
telephone	O
line	O
attached	O
to	O
a	O
private	B-Device
branch	I-Device
exchange	I-Device
(	O
PBX	O
)	O
or	O
Centrex	B-Device
system	O
.	O
</s>
<s>
In	O
these	O
systems	O
,	O
one	O
usually	O
has	O
to	O
dial	O
a	O
number	O
(	O
typically	O
9	O
in	O
North	O
America	O
,	O
0	O
in	O
Europe	O
)	O
to	O
tell	O
the	O
PBX	O
to	O
connect	O
with	O
an	O
outside	O
landline	O
(	O
also	O
called	O
DDCO	O
,	O
or	O
Direct	B-Device
Dial	I-Device
Central	I-Device
Office	I-Device
)	O
to	O
dial	O
an	O
external	O
number	O
.	O
</s>
<s>
For	O
inbound	O
calls	O
,	O
a	O
switchboard	O
operator	O
or	O
automated	O
attendant	O
may	O
request	O
the	O
number	O
of	O
the	O
desired	O
extension	O
or	O
the	O
call	O
may	O
be	O
completed	O
with	O
direct	B-Device
inbound	I-Device
dialing	I-Device
,	O
if	O
outside	O
numbers	O
are	O
assigned	O
to	O
individual	O
extensions	O
.	O
</s>
<s>
An	O
off-premises	O
extension	O
,	O
where	O
a	O
worker	O
at	O
a	O
remote	O
location	O
employs	O
a	O
telephone	O
configured	O
to	O
appear	O
as	O
if	O
it	O
were	O
an	O
extension	O
located	O
at	O
the	O
main	O
business	O
site	O
,	O
may	O
be	O
created	O
in	O
analog	O
telephony	O
by	O
using	O
a	O
leased	B-Architecture
line	I-Architecture
to	O
connect	O
the	O
extension	O
to	O
the	O
main	O
enterprise	O
system	O
.	O
</s>
<s>
Voice	B-Application
over	I-Application
IP	I-Application
makes	O
the	O
creation	O
of	O
off-premises	O
extensions	O
inexpensive	O
and	O
trivial	O
as	O
broadband	O
Internet	O
and	O
virtual	B-Application
private	I-Application
networking	I-Application
can	O
extend	O
local	O
network	O
access	O
anywhere	O
in	O
the	O
world	O
.	O
</s>
<s>
Extensions	O
allow	O
personalized	O
traffic	O
and	O
call	B-Application
accounting	I-Application
data	O
to	O
be	O
captured	O
by	O
the	O
PBX	O
or	O
a	O
phone	O
recorder	O
for	O
cost	O
control	O
.	O
</s>
<s>
A	O
hotel	O
or	O
motel	O
,	O
for	O
example	O
,	O
could	O
install	O
PBX	O
extensions	O
in	O
each	O
individual	O
room	O
and	O
use	O
the	O
private	B-Device
branch	I-Device
exchange	I-Device
's	O
call	B-Application
accounting	I-Application
to	O
generate	O
individualised	O
long-distance	O
bills	O
for	O
each	O
room	O
that	O
are	O
presented	O
to	O
the	O
guest	O
upon	O
check-out	O
.	O
</s>
