[Asterisk-Users] yet another question on DID trunks

[david]

> I'm still trying to examine a DID solution for this customer but don't
> understand how a single trunk (whatever that is--I assumed just a single
> pair of wires like a POTS line, but I'm thinking now it must not be) can
> support multiple (incoming) phones calls.

As another attempt to help clarify this, I can add that DID is a service you subscribe to with your local carrier as a range of numbers that are shared over a smaller number of physical DID trunks brought into your premise by the carrier.  For example, you might buy a block of 50 numbers ranging from 650-555-5200 to 650-555-5249 and subscribe to 5 DID circuits.  

A special signalling method over these trunks delivers the actual number that was dialed (the last 4 digits) to the PBX on each inbound call. This is not the same as ANI.  This is information on the called number, not the calling number.   The PBX then interprets the dialed number information and rings the proper station.  That is a general description of how the classic DID service works over inband/analog signalling.  Each DID circuit can only handle 1 incoming call at a time.  Pure DID circuits are used only for inbound calls, not outbound.

Variations on this exist in the digital signalling to support the same result (ringing a specific phone with 10-digits over shared trunks).  You can do this with ISDN or T1 or Inward 800 service (aka DNIS) or other means.  In some of these cases, the same lines may be used for 2-way traffic (inbound and outbound).  None of these are the same services from the carrier and don't interface with the PBX the same way. They all have a much different pricing structure ranging from, say, $50/mo per DID line, to hundreds or thousands per month for T1 or ISDN PRI (24 or 23 channels).