This will return details of each call currently in progress in order of their Call ID number. The
second line of each entry will list the Call Serial Number, and the third will list the Call Tag.

Call ID

Call serial number

Each call that passes through the VCS is assigned a Call ID and a Call Serial Number. Calls also
have a Call Tag assigned if this does not already exist.

Call ID

The VCS assigns each call currently in progress a different Call ID. The Call ID numbers start at 1
and go up to the maximum number of calls allowed on that system.
Each time a call is made, the VCS will assign that call the lowest available Call ID number. For
example, if there is already a call in progress with a Call ID of 1, the next call will be assigned a
Call ID of 2. If Call 1 is then disconnected, the third call to be made will be assigned a Call ID of 1.
The Call ID is not therefore a unique identifier: while no two calls in progress at the same time will
have the same Call ID, the same Call ID will be assigned to more than one call over time.

Call Serial Number

The VCS assigns a unique Call Serial Number to every call passing through it. No two calls on a
VCS will ever have the same Call Serial Number. However, a single call passing between two or
more VCSs will be identified by a different Call Serial Number on each system.

Call Tag

Call Tags are used to track calls passing through a number of VCSs. When the VCS receives a call,
it checks to see if there is a Call Tag already assigned to it. If so, the VCS will use the existing
Call Tag; if not, it will assign a new Call Tag to the call. This Call Tag is then included in the call’s
details when the call is forwarded on. A single call passing between two or more VCSs will be
assigned a different Call Serial Number each time it arrives at a VCS (including one it has already
passed through) but can be identified as the same call by use of the Call Tag. This is particularly
useful if you are

using a remote syslog server

to collate events across a number of VCSs in your

network.
The Call Tag also helps you identify loops in your network. Loops occur when a query is sent to a
neighbor zone and passes through one or more systems before being routed back to the original
VCS. In this situation the outgoing and incoming query will have different Call Serial Numbers
and may even be for different destination aliases (depending on whether any transforms had been
applied). However, the call will still have the same Call Tag.

Call Tags are supported by VCS version X3.0 and above. If a call passes through a system
that is not a VCS, or a VCS that is running an earlier version of the software, the Call Tag
information will be lost.

Call tag