Programming Instructions
8.4
Inward Dialing (DID) Support
The DID line board allows incoming CO calls to reach internal DXP
intercom extensions by direct dialing.
No attendant assistance is necessary. Since DID lines are incoming only, their direct appearance is limited to
attendant stations where status indication may be useful. DID operation requires a group of published directory
numbers (400 maximum) provided by the telephone company central office (CO). These directory numbers are
incoming only and the DXP
translates them to the appropriate personal or group intercom number for
ringing. The lines will accept outgoing DTMF digits during a call to support personal identification number
dialing and similar uses.
The DXP
Plus
handles DID calls in the same manner that it handles regular intercom calls and will forward them
via a hunt list or a call forwarding scheme. An unanswered DID call will either continue to ring or route to a
ring no-answer (RNA) destination. If neither forwarding nor RNA is available, the system provides
no routing.
The system returns a
signal to DID callers when a called station is in the do not disturb (DND) mode.
The system returns an off-hook indication to the CO when a DID line is disabled. The
Plus
synchronized
ringing feature does not have any affect on DID calls, and the Caller ID feature is not available.
The DID line is a reverse-battery, loop-start, two-wire voice circuit.
(The term reverse battery means that it is the
DXP
Plus
and not the CO that provides tip and ring supervisory battery for the loop.)
The DXP
sends and
receives all supervisory and voice signals across the voice pair and provides all necessary signal control at its line
interface.
In the idle state, the loop is open at the CO end, with the
Plus
maintaining on-hook battery polarity on the
circuit. The CO requests service from the
Plus
by applying a resistance across te line causing a current flow
to occur. The DXP
recognizes the current flow as a CO connection and prepares for the incoming call. The
CO does not send
since the DXP
Plus
generates it internally and sends it to the stations.
After the DXP
receives all of the address digits, it translates them based on the appropriate DID block
translation table and attempts to place the call to the desired station. To assure that the caller will always hear
prior to the call being answered (for example, when answered by voice mail), the system delays the
station ringing until it applies
tone.
The called station sounds a distinctive ringing based on the ring code in the translation table.
If the translation
table has a name for the CO digit sequence that the DID call is on, the LCD of the called station displays that
name. Otherwise, the station displays the DID block name followed by a portion of the CO digit string. If there is
neither CO digit string name nor DID block name programmed, the station displays the CO digit string alone.
If an incoming call is for either an unassigned or an uninstalled station, the DXP
routes the call to the DID
block alternate extension. If the DID Block alternate extension is unavailable, the DXP
routes the call to the
Dial 0 attendant. If the Dial 0 attendant is unavailable, the DXP
Plus
returns reorder tone to the caller.
The DXP
Plus
returns call progress signals to the CO in the interval after it receives address signalling and before
it answers. It does this within 1.5 seconds after completion of address signalling.
When a called station answers a DID call, the DXP
Plus
sends answer supervision to the CO by applying reverse
polarity to the line. The DXP
Plus
maintains this polarity reversal until either the station goes on-hook or the CO
opens the loop.
The CO signals a line disconnect condition to the
Plus
by opening the loop. When the
Plus
detects the
CO disconnect, it returns the line to idle polarity and changes the line from busy to idle. The DXP
Plus
signals a
line disconnect condition to the CO by returning the supervision battery polarity to the on-hook state. While
immediate start lines return to idle at once, wink and dela start lines maintain busy status until the DXP
detects CO release.
6-14 Line Features Programming
