<s>
A	O
business	O
telephone	O
system	O
is	O
a	O
multiline	O
telephone	O
system	O
typically	O
used	O
in	O
business	O
environments	O
,	O
encompassing	O
systems	O
ranging	O
in	O
technology	O
from	O
the	O
key	O
telephone	O
system	O
(	O
KTS	O
)	O
to	O
the	O
private	B-Device
branch	I-Device
exchange	I-Device
(	O
PBX	O
)	O
.	O
</s>
<s>
Business	O
telephone	O
systems	O
are	O
often	O
broadly	O
classified	O
into	O
key	O
telephone	O
systems	O
,	O
and	O
private	B-Device
branch	I-Device
exchanges	I-Device
,	O
but	O
many	O
hybrid	O
systems	O
exist	O
.	O
</s>
<s>
A	O
key	O
telephone	O
system	O
was	O
originally	O
distinguished	O
from	O
a	O
private	B-Device
branch	I-Device
exchange	I-Device
in	O
that	O
it	O
did	O
not	O
require	O
an	O
operator	O
or	O
attendant	O
at	O
the	O
switchboard	B-Device
to	O
establish	O
connections	O
between	O
the	O
central	O
office	O
trunks	O
and	O
stations	O
,	O
or	O
between	O
stations	O
.	O
</s>
<s>
Technologically	O
,	O
private	B-Device
branch	I-Device
exchanges	I-Device
share	O
lineage	O
with	O
central	O
office	O
telephone	O
systems	O
,	O
and	O
in	O
larger	O
or	O
more	O
complex	O
systems	O
,	O
may	O
rival	O
a	O
central	O
office	O
system	O
in	O
capacity	O
and	O
features	O
.	O
</s>
<s>
New	O
installations	O
of	O
key	O
telephone	O
systems	O
have	O
become	O
less	O
common	O
,	O
as	O
hybrid	O
systems	O
and	O
private	B-Device
branch	I-Device
exchanges	I-Device
of	O
comparable	O
size	O
have	O
similar	O
cost	O
and	O
greater	O
functionality	O
.	O
</s>
<s>
The	O
modern	O
key	O
system	O
now	O
supports	O
SIP	B-Application
,	O
ISDN	B-Protocol
,	O
analog	O
handsets	O
(	O
in	O
addition	O
to	O
its	O
own	O
proprietary	O
handsets	O
-	O
usually	O
digital	O
)	O
as	O
well	O
as	O
a	O
raft	O
of	O
features	O
more	O
traditionally	O
found	O
on	O
larger	O
PBX	O
systems	O
.	O
</s>
<s>
The	O
modern	O
key	O
system	O
is	O
usually	O
fully	O
digital	O
,	O
although	O
analog	O
variants	O
persist	O
and	O
some	O
systems	O
implement	O
VOIP	B-Application
services	O
.	O
</s>
<s>
Each	O
device	O
connected	O
to	O
the	O
PBX	O
,	O
such	O
as	O
a	O
telephone	O
,	O
a	O
fax	O
machine	O
,	O
or	O
a	O
computer	O
modem	O
,	O
is	O
referred	O
to	O
as	O
an	O
extension	B-Application
and	O
has	O
a	O
designated	O
extension	B-Application
telephone	O
number	O
that	O
may	O
or	O
may	O
not	O
be	O
mapped	O
automatically	O
to	O
the	O
numbering	O
plan	O
of	O
the	O
central	O
office	O
and	O
the	O
telephone	O
number	O
block	O
allocated	O
to	O
the	O
PBX	O
.	O
</s>
<s>
Initially	O
,	O
PBX	O
systems	O
offered	O
the	O
primary	O
advantage	O
of	O
cost	O
savings	O
for	O
internal	O
phone	O
calls	O
:	O
handling	O
the	O
circuit	B-Protocol
switching	I-Protocol
locally	O
reduced	O
charges	O
for	O
telephone	O
service	O
via	O
central-office	O
lines	O
.	O
</s>
<s>
As	O
PBX	O
systems	O
gained	O
popularity	O
,	O
they	O
began	O
to	O
feature	O
services	O
not	O
available	O
in	O
the	O
public	O
network	O
,	O
such	O
as	O
hunt	O
groups	O
,	O
call	O
forwarding	O
,	O
and	O
extension	B-Application
dialing	O
.	O
</s>
<s>
From	O
the	O
1960s	O
,	O
a	O
simulated	O
PBX	O
,	O
known	O
as	O
Centrex	B-Device
,	O
provided	O
similar	O
features	O
from	O
the	O
central	O
telephone	O
exchange	O
.	O
</s>
<s>
The	O
term	O
PBX	O
originated	O
when	O
switchboard	B-Device
operators	O
managed	O
company	O
switchboards	O
manually	O
using	O
cord	O
circuits	O
.	O
</s>
<s>
One	O
was	O
the	O
massive	O
growth	O
of	O
data	O
networks	O
and	O
increased	O
public	O
understanding	O
of	O
packet	B-Protocol
switching	I-Protocol
.	O
</s>
<s>
Companies	O
needed	O
packet-switched	B-Protocol
networks	I-Protocol
for	O
data	O
,	O
so	O
using	O
them	O
for	O
telephone	O
calls	O
proved	O
tempting	O
,	O
and	O
the	O
availability	O
of	O
the	O
Internet	O
as	O
a	O
global	O
delivery	O
system	O
made	O
packet-switched	B-Protocol
communications	O
even	O
more	O
attractive	O
.	O
</s>
<s>
These	O
factors	O
led	O
to	O
the	O
development	O
of	O
the	O
voice	B-Application
over	I-Application
IP	I-Application
PBX	O
,	O
or	O
IP-PBX	O
.	O
</s>
<s>
In	O
wireline	O
telephony	O
,	O
the	O
original	O
hosted	O
PBX	O
was	O
the	O
Centrex	B-Device
service	I-Device
provided	O
by	O
telcos	O
since	O
the	O
1960s	O
;	O
later	O
competitive	O
offerings	O
evolved	O
into	O
the	O
modern	O
competitive	O
local	O
exchange	O
carrier	O
.	O
</s>
<s>
In	O
voice	B-Application
over	I-Application
IP	I-Application
,	O
hosted	O
solutions	O
are	O
easier	O
to	O
implement	O
as	O
the	O
PBX	O
may	O
be	O
located	O
at	O
and	O
managed	O
by	O
any	O
telephone	O
service	O
provider	O
,	O
connecting	O
to	O
the	O
individual	O
extensions	O
via	O
the	O
Internet	O
.	O
</s>
<s>
Many	O
manufacturers	O
provided	O
manually	O
operated	O
private	B-Device
branch	I-Device
exchange	I-Device
systems	O
in	O
various	O
sizes	O
and	O
features	O
;	O
examples	O
are	O
pictured	O
here	O
:	O
</s>
<s>
Console	O
or	O
switchboard	B-Device
allows	O
the	O
operator	O
to	O
control	O
incoming	O
calls	O
.	O
</s>
<s>
Microcontroller	B-Architecture
or	O
microcomputer	B-Architecture
for	O
arbitrary	O
data	O
processing	O
,	O
control	O
and	O
logic	O
.	O
</s>
<s>
Since	O
the	O
advent	O
of	O
Internet	B-Application
telephony	I-Application
(	O
Voice	B-Application
over	I-Application
IP	I-Application
)	O
technologies	O
,	O
PBX	O
development	O
has	O
tended	O
toward	O
the	O
IP	O
PBX	O
,	O
which	O
uses	O
the	O
Internet	B-Protocol
Protocol	I-Protocol
to	O
carry	O
calls	O
.	O
</s>
<s>
Most	O
modern	O
PBXs	O
support	O
VoIP	B-Application
.	O
</s>
<s>
ISDN	B-Protocol
PBX	O
systems	O
also	O
replaced	O
some	O
traditional	O
PBXs	O
in	O
the	O
1990s	O
,	O
as	O
ISDN	B-Protocol
offers	O
features	O
such	O
as	O
conference	O
calling	O
,	O
call	O
forwarding	O
,	O
and	O
programmable	O
caller	O
ID	O
.	O
</s>
<s>
As	O
of	O
2015	O
ISDN	B-Protocol
is	O
being	O
phased	O
out	O
by	O
most	O
major	O
telecommunication	O
carriers	O
throughout	O
Europe	O
in	O
favour	O
of	O
all-IP	O
networks	O
,	O
with	O
some	O
expecting	O
complete	O
migration	O
by	O
2025	O
.	O
</s>
<s>
Originally	O
having	O
started	O
as	O
an	O
organization	O
's	O
manual	O
switchboard	B-Device
or	O
attendant	O
console	O
operated	O
by	O
a	O
telephone	O
operator	O
or	O
just	O
simply	O
the	O
operator	O
,	O
PBXs	O
have	O
evolved	O
into	O
VoIP	B-Application
centers	O
that	O
are	O
hosted	O
by	O
the	O
operators	O
or	O
even	O
manufacturers	O
.	O
</s>
<s>
Even	O
though	O
VoIP	B-Application
is	O
considered	O
by	O
many	O
people	O
as	O
the	O
future	O
of	O
telephony	O
,	O
the	O
circuit	B-Protocol
switched	I-Protocol
network	I-Protocol
remains	O
the	O
core	O
of	O
communications	O
,	O
and	O
the	O
existing	O
PBX	O
systems	O
are	O
competitive	O
in	O
services	O
with	O
modern	O
IP	O
systems	O
.	O
</s>
<s>
For	O
the	O
option	O
to	O
call	O
from	O
IP	O
network	O
to	O
the	O
circuit-switched	B-Protocol
PSTN	O
(	O
SS7/ISUP	O
)	O
,	O
the	O
hosted	O
solutions	O
include	O
interconnecting	O
media	O
gateways	O
.	O
</s>
<s>
In	O
Europe	O
these	O
systems	O
for	O
analog	O
phones	O
were	O
followed	O
by	O
consumer-grade	O
PBXs	O
for	O
ISDN	B-Protocol
.	O
</s>
<s>
Using	O
small	O
PBXs	O
for	O
ISDN	B-Protocol
is	O
a	O
logical	O
step	O
,	O
since	O
the	O
ISDN	B-Protocol
basic	B-Protocol
rate	I-Protocol
interface	I-Protocol
provides	O
two	O
logical	O
phone	O
lines	O
(	O
via	O
two	O
ISDN	B-Protocol
B	O
channels	O
)	O
which	O
can	O
be	O
used	O
in	O
parallel	O
.	O
</s>
<s>
With	O
the	O
adoption	O
of	O
VoIP	B-Application
by	O
consumers	O
,	O
consumer	O
VoIP	B-Application
PBXs	O
have	O
appeared	O
,	O
with	O
PBX	O
functions	O
becoming	O
simple	O
additional	O
software	B-Architecture
features	I-Architecture
of	O
consumer-grade	O
routers	B-Protocol
and	O
switches	O
.	O
</s>
<s>
Open	B-License
source	I-License
projects	O
have	O
provided	O
PBX-style	O
features	O
since	O
the	O
1990s	O
.	O
</s>
<s>
IP-PBX	O
is	O
a	O
PBX	O
phone	O
system	O
that	O
uses	O
Internet	B-Protocol
Protocol	I-Protocol
(	O
IP	O
)	O
data	O
networks	O
to	O
manage	O
the	O
routing	O
and	O
switching	O
of	O
calls	O
as	O
well	O
as	O
to	O
handle	O
messaging	O
.	O
</s>
<s>
Apart	O
from	O
offering	O
advanced	O
features	O
(	O
like	O
voicemail	O
)	O
,	O
the	O
system	O
could	O
also	O
use	O
a	O
VoIP	B-Protocol
gateway	I-Protocol
to	O
connect	O
to	O
traditional	O
PSTN	O
lines	O
.	O
</s>
<s>
DECT	B-Protocol
–	O
a	O
standard	O
for	O
connecting	O
cordless	O
phones	O
.	O
</s>
<s>
Internet	B-Protocol
Protocol	I-Protocol
–	O
For	O
example	O
,	O
H.323	B-Protocol
and	O
SIP	B-Application
.	O
</s>
<s>
This	O
is	O
cheap	O
and	O
effective	O
,	O
and	O
allows	O
almost	O
any	O
standard	O
phone	O
to	O
be	O
used	O
as	O
an	O
extension	B-Application
.	O
</s>
<s>
Internet	B-Protocol
Protocol	I-Protocol
–	O
H.323	B-Protocol
and	O
the	O
Session	B-Application
Initiation	I-Application
Protocol	I-Application
(	O
SIP	B-Application
)	O
are	O
IP-based	O
solutions	O
for	O
multimedia	O
sessions	O
.	O
</s>
<s>
Primary	B-Protocol
rate	I-Protocol
interface	I-Protocol
(	O
ISDN	B-Protocol
)	O
–	O
Provided	O
over	O
T1	O
(	O
23	O
bearer	O
channels	O
and	O
1	O
signaling	O
channel	O
)	O
or	O
E1	O
carriers	O
.	O
</s>
<s>
Internet	B-Protocol
Protocol	I-Protocol
–	O
H.323	B-Protocol
,	O
SIP	B-Application
,	O
MGCP	B-Protocol
,	O
and	O
Inter-Asterisk	B-Protocol
eXchange	I-Protocol
protocols	O
operate	O
over	O
IP	O
and	O
are	O
supported	O
by	O
some	O
network	O
providers	O
.	O
</s>
<s>
ISDN	B-Protocol
–	O
the	O
most	O
common	O
digital	O
standard	O
for	O
fixed	O
telephony	O
devices	O
.	O
</s>
<s>
Most	O
medium	O
to	O
large	O
companies	O
would	O
use	O
Primary	O
ISDN	B-Protocol
circuits	O
carried	O
on	O
T1	O
or	O
E1	O
physical	O
connections	O
.	O
</s>
<s>
Network	B-Protocol
port	I-Protocol
(	O
listen	O
mode	O
)	O
–	O
an	O
external	O
application	O
connects	O
to	O
the	O
TCP	B-Protocol
or	O
UDP	B-Protocol
port	B-Protocol
.	O
</s>
<s>
Network	B-Protocol
port	I-Protocol
(	O
server	O
mode	O
)	O
–	O
the	O
PBX	O
connects	O
to	O
another	O
application	O
or	O
buffer	O
.	O
</s>
<s>
Serial	B-Protocol
interface	O
–	O
historically	O
used	O
to	O
print	O
every	O
call	O
record	O
to	O
a	O
serial	B-Protocol
printer	O
.	O
</s>
<s>
In	O
modern	O
systems	O
a	O
software	O
application	O
connects	O
via	O
serial	B-Protocol
cable	O
to	O
this	O
port	B-Protocol
.	O
</s>
<s>
The	O
first	O
hosted	O
PBX	O
services	O
were	O
feature-rich	B-Architecture
compared	O
to	O
most	O
premises-based	O
systems	O
of	O
the	O
time	O
.	O
</s>
<s>
allows	O
multimodal	O
access	O
,	O
where	O
employees	O
access	O
the	O
network	O
via	O
a	O
variety	O
of	O
telecommunications	O
systems	O
,	O
including	O
POTS	O
,	O
ISDN	B-Protocol
,	O
cellular	O
phones	O
,	O
and	O
VOIP	B-Application
.	O
</s>
<s>
This	O
allows	O
one	O
extension	B-Application
to	O
ring	O
in	O
multiple	O
locations	O
(	O
either	O
concurrently	O
or	O
sequentially	O
)	O
.	O
</s>
<s>
The	O
ongoing	O
migration	O
of	O
most	O
major	O
telecommunication	O
carriers	O
to	O
IP-based	O
networks	O
,	O
coupled	O
with	O
the	O
rise	O
in	O
Cloud	B-General_Concept
Communications	I-General_Concept
has	O
resulted	O
in	O
a	O
significant	O
rise	O
in	O
the	O
uptake	O
of	O
hosted	O
PBX	O
solutions	O
.	O
</s>
<s>
In	O
addition	O
,	O
a	O
mobile	O
PBX	O
may	O
create	O
extension	B-Application
identifiers	O
for	O
each	O
handset	O
that	O
allow	O
to	O
dial	O
other	O
cell	O
phones	O
in	O
the	O
PBX	O
via	O
their	O
extension	B-Application
shortcut	O
,	O
instead	O
of	O
a	O
PSTN	O
number	O
.	O
</s>
<s>
An	O
IP	O
PBX	O
handles	O
voice	O
signals	O
over	O
Internet	B-Protocol
protocol	I-Protocol
,	O
bringing	O
benefits	O
for	O
computer	O
telephony	O
integration	O
(	O
CTI	O
)	O
.	O
</s>
