DID (Direct Inward Dial)

This term is used loosely and confuses many.  The concept is simple, but there are a lot of caveats.  

DID and non-DID are used to describe two types of phones in a building or campus (PBX telephone system).  This term is used as a discriminator to differentiate the two types of phones with a PBX or Centrex system.

• DID numbers can be directly dialed from outside (and inside)
    
• Non-DID numbers cannot be called directly from outside.  Outside callers will need to call a switchboard operator who will connect you.

DID simply means that the outside caller can dial your office phone directly  (an inward dialed call - direct to your phone).  The only other scenario, "non-DID, would be for an attendant to answer all calls, and connect you to a user's extension.

With DID, people can call you directly by dialing your 10-digit number.  If they are in the same system then they can usually call you by dialing your last 4 digits, or a specific lead digit, such as "5" and then your last 4 digits.

DID trunks are usually one way (in-bound) ! ! !

However, DID can work over 2-way trunks as well.  It is a rather sophisticated service and must be set up carefully with the phone company.  The reason for this complexity, is that a small number of trunks support a large number of stations.  For example, imagine a hospital, that has 500 patient beds and 400 administrators - each with one phone.  They need 900 phone numbers, but of course would need extras, so they order 1000 numbers.  They do not need 1000 trunks since only a fraction of the people are talking on the phone at any one time.  So they order 40 DID trunks.

DID Options - a few things that must be provided and agreed upon by the PBX and CO :

• answer supervision  -  When a DID call comes in from the network, it goes directly to
  the station set with that DID number. When a person answers, the PBX must tell the CO that the call has been
  answered.  When the person hangs up, the PBX must tell the CO that the call has ended.
• start signaling & options  (Loop or Ground Start Signaling - with Wink, Immediate, or Delay start options)
• number of digits to pulse - 2 to 7 digits
• type of pulsing to use - there are 3 types as follows :

1. Dial Pulse (DP) - the PBX accepts the DID digits as make/break pulses used by
   rotary dial phones.
2. Dual Tone Multi-Frequency (DTMF) - the PBX accepts the DID digits as tones
   used by push-button phones. 
3. Multi-Frequency (MF) - the PBX accepts the DID digits as tones that many CO
   switches use to communicate within the network. This is very uncommon. When
   a customer requests MF signaling, check to be sure that the request is accurate.

DID Blocks - customers usually like their DID numbers to be in sequence. This can be difficult. In many areas so many phone numbers have been assigned that long sequences are hard to come by.  In general, DID numbers are grouped in blocks of 20. If a customer requires sequential numbering, but only requires a few numbers, they usually must order a full block of 20, but this depends on the carrier and their rules. On rare occasions, customers may order single DID numbers that are available from the POTS number pool.  If a customer wants to break a number out of the block, there is an additional block compromise charge.  The customer can, without charge, break a number out of the block for billed number screening.  A customer moving from one location to another can also break the block without charge. During the move, some numbers will be in use in one location and some in the other. When the move is complete, the block will be back together for service at the new location.  

In-Only or 2-Way DID Trunks?  -  most DID trunks are in-only. You can also, however, have DID on two-way trunks. The choice of in-only or two-way depends on: 

• your inward and outward call volumes
• the importance of completing DID calls, and
• the relative cost of in-only and two-way trunks.

Inward DID call on two-way trunk - these calls work like any other DID call. The CO switch sends the digit pulses that the PBX uses to complete the call.

Outward DID call on two-way trunk - these work like any other outward call. The PBX sends the start signal to the CO switch. The CO returns the dial tone and the user dials the phone number.

DID Directory Listings - usually, only a company's main phone number is listed in the telephone book. As an option,
the company can list DID numbers separately. DID customers who want to give their callers easy access will often use optional DID listings for departmental numbers along with main numbers.