<s>
Direct	B-Device
inward	I-Device
dialing	I-Device
(	O
DID	O
)	O
,	O
also	O
called	O
direct	B-Device
dial-in	I-Device
(	O
DDI	O
)	O
in	O
Europe	O
and	O
Oceania	O
,	O
is	O
a	O
telecommunication	O
service	O
offered	O
by	O
telephone	O
companies	O
to	O
subscribers	O
who	O
operate	O
a	O
private	B-Device
branch	I-Device
exchange	I-Device
(	O
PBX	B-Device
)	O
system	O
.	O
</s>
<s>
The	O
feature	O
provides	O
service	O
for	O
multiple	O
telephone	O
numbers	O
over	O
one	O
or	O
more	O
analog	O
or	O
digital	O
physical	O
circuits	O
to	O
the	O
PBX	B-Device
,	O
and	O
transmits	O
the	O
dialed	O
telephone	O
number	O
to	O
the	O
PBX	B-Device
so	O
that	O
a	O
PBX	B-Device
extension	O
is	O
directly	O
accessible	O
for	O
an	O
outside	O
caller	O
,	O
possibly	O
by-passing	O
an	O
auto-attendant	O
.	O
</s>
<s>
For	O
direct	B-Device
inward	I-Device
dialing	I-Device
service	O
,	O
the	O
telephone	O
company	O
provides	O
one	O
or	O
more	O
trunk	O
lines	O
to	O
the	O
customer	O
for	O
connection	O
to	O
the	O
customer	O
's	O
PBX	B-Device
,	O
and	O
allocates	O
a	O
range	O
of	O
telephone	O
numbers	O
to	O
the	O
customer	O
.	O
</s>
<s>
Calls	O
to	O
such	O
numbers	O
are	O
forwarded	O
to	O
the	O
customer	O
's	O
PBX	B-Device
via	O
the	O
trunks	O
.	O
</s>
<s>
As	O
calls	O
are	O
presented	O
to	O
the	O
PBX	B-Device
,	O
the	O
dialed	O
telephone	O
number	O
is	O
signaled	O
to	O
the	O
PBX	B-Device
with	O
Dialed	O
Number	O
Identification	O
Service	O
(	O
DNIS	O
)	O
using	O
a	O
prearranged	O
,	O
usually	O
partial	O
format	O
,	O
e.g.	O
,	O
the	O
last	O
four	O
digits	O
.	O
</s>
<s>
The	O
PBX	B-Device
may	O
use	O
this	O
information	O
to	O
route	O
the	O
call	O
directly	O
to	O
a	O
telephone	B-Application
extension	I-Application
within	O
the	O
organization	O
without	O
the	O
need	O
for	O
an	O
operator	O
or	O
attendant	O
.	O
</s>
<s>
Nowadays	O
,	O
it	O
is	O
far	O
more	O
common	O
to	O
deliver	O
DID	O
service	O
on	O
a	O
Primary	B-Protocol
Rate	I-Protocol
Interface	I-Protocol
(	O
PRI	O
)	O
circuit	O
.	O
</s>
<s>
The	O
trunks	O
for	O
DID	O
service	O
are	O
unidirectional	O
,	O
inbound	O
to	O
the	O
customer	O
PBX	B-Device
.	O
</s>
<s>
However	O
,	O
the	O
service	O
may	O
be	O
combined	O
with	O
direct	O
outward	O
dialing	O
(	O
DOD	O
)	O
allowing	O
PBX	B-Device
extensions	O
direct	O
outbound	O
calling	O
capability	O
with	O
identification	O
of	O
their	O
DID	O
telephone	O
number	O
.	O
</s>
<s>
DID	O
service	O
is	O
also	O
used	O
by	O
fax	B-Application
servers	I-Application
.	O
</s>
<s>
A	O
telephone	O
line	O
is	O
terminated	O
at	O
a	O
telephone	O
interface	O
(	O
fax	O
modem	O
)	O
of	O
a	O
computer	O
that	O
runs	O
fax	B-Application
server	I-Application
software	O
.	O
</s>
<s>
Some	O
voice	B-Application
over	I-Application
IP	I-Application
(	O
VoIP	B-Application
)	O
vendors	O
have	O
used	O
one	O
central	O
,	O
remotely	O
located	O
fax	B-Application
server	I-Application
as	O
a	O
means	O
of	O
offering	O
Internet	O
fax	O
service	O
to	O
their	O
clients	O
.	O
</s>
<s>
In	O
theory	O
,	O
standards	O
such	O
as	O
T.38	B-Protocol
should	O
have	O
allowed	O
VoIP	B-Application
subscribers	O
to	O
keep	O
their	O
existing	O
fax	O
equipment	O
working	O
locally	O
;	O
in	O
practice	O
,	O
T.38	B-Protocol
at	O
the	O
subscriber	O
's	O
site	O
offers	O
no	O
benefit	O
if	O
the	O
upstream	O
provider	O
is	O
least-cost	O
routing	O
to	O
gateways	B-Application
that	O
do	O
not	O
support	O
T.38	B-Protocol
and	O
cannot	O
reliably	O
send	O
or	O
receive	O
fax/modem	O
traffic	O
.	O
</s>
<s>
A	O
fax	B-Application
server	I-Application
at	O
a	O
central	O
location	O
,	O
connected	O
directly	O
to	O
public	O
switched	O
telephone	O
network	O
(	O
PSTN	O
)	O
T-carrier	O
primary	B-Protocol
rate	I-Protocol
interface	I-Protocol
lines	O
and	O
using	O
direct	B-Device
inward	I-Device
dial	I-Device
to	O
identify	O
the	O
intended	O
addressee	O
can	O
convert	O
an	O
incoming	O
fax	O
to	O
an	O
electronic	O
document	O
(	O
such	O
as	O
.tif	O
or	O
.pdf	B-Application
)	O
for	O
web	O
or	O
e-mail	O
delivery	O
.	O
</s>
<s>
The	O
fax	O
traffic	O
never	O
passes	O
through	O
the	O
VoIP	B-Application
infrastructure	O
as	O
a	O
dial-up	O
modem	O
call	O
and	O
therefore	O
arrives	O
reliably	O
even	O
if	O
T.38	B-Protocol
is	O
not	O
properly	O
supported	O
at	O
some	O
points	O
in	O
the	O
network	O
.	O
</s>
<s>
DID	O
service	O
has	O
similar	O
relevance	O
for	O
Voice	B-Application
over	I-Application
Internet	I-Application
Protocol	I-Application
(	O
VoIP	B-Application
)	O
communications	O
.	O
</s>
<s>
To	O
allow	O
public	O
switched	O
telephone	O
network	O
(	O
PSTN	O
)	O
users	O
to	O
directly	O
reach	O
users	O
with	O
VoIP	B-Application
phones	O
,	O
DID	O
numbers	O
are	O
assigned	O
to	O
a	O
communications	B-Application
gateway	I-Application
.	O
</s>
<s>
The	O
gateway	B-Application
connects	O
the	O
PSTN	O
to	O
the	O
VoIP	B-Application
network	O
,	O
routing	O
and	O
translating	O
calls	O
between	O
the	O
two	O
networks	O
.	O
</s>
<s>
For	O
voice	B-Application
over	I-Application
IP	I-Application
resellers	O
,	O
some	O
specialized	O
CLECs	O
(	O
for	O
local	O
numbers	O
)	O
or	O
interexchange	O
carriers	O
(	O
for	O
toll-free	O
numbers	O
)	O
will	O
deliver	O
blocks	O
of	O
direct	B-Device
inward	I-Device
dial	I-Device
calls	O
already	O
converted	O
to	O
Session	B-Application
Initiation	I-Application
Protocol	I-Application
(	O
SIP	O
)	O
or	O
common	O
VoIP	B-Application
formats	O
.	O
</s>
<s>
The	O
individual	O
VoIP	B-Application
provider	O
need	O
only	O
obtain	O
an	O
inventory	O
of	O
local	O
or	O
freephone	O
numbers	O
from	O
VoIP-aware	O
carriers	O
in	O
various	O
regions	O
,	O
import	O
them	O
in	O
bulk	O
to	O
an	O
IP	O
PBX	B-Device
and	O
issue	O
them	O
individually	O
to	O
end	O
users	O
.	O
</s>
<s>
The	O
corresponding	O
service	O
to	O
DID	O
for	O
outgoing	O
calls	O
from	O
a	O
PBX	B-Device
to	O
the	O
central	O
office	O
exchange	O
is	O
called	O
direct	O
outward	O
dialing	O
(	O
DOD	O
)	O
or	O
Direct	B-Device
Dial	I-Device
Central	I-Device
Office	I-Device
(	O
DDCO	O
)	O
.	O
</s>
