The VCS is able to act as a gateway between SIP and H.323, translating calls from one protocol to
the other. This is known as “interworking”.
By default, the VCS acts as a SIP-H.323 and H.323-SIP gateway but only if at least one of the
endpoints that are involved in the call is locally registered. You can change this setting so that
the VCS will act as a SIP-H.323 gateway regardless of whether the endpoints involved are locally
registered. You also have the option to disable interworking completely.

The VCS uses the protocol of the incoming call when searching a zone for a given alias. If
the search is unsuccessful the VCS may then search the same zone again using the
alternative protocol, depending on where the search came from and the Interworking mode
(VCS configuration > Protocols > Interworking).

If the request has come from a neighboring system and Interworking mode is set to
RegisteredOnly, the VCS will search the Local Zone using both protocols, and all other zones using
the native protocol only (because it will interwork the call only if one of the endpoints is locally
registered). If Interworking mode is set to On, or the request has come from a locally registered
endpoint, the VCS will search the Local Zone and all external zones using both protocols.

Calls for which the VCS acts as a SIP to H.323 gateway are traversal calls. They will
therefore require a traversal call license.

Overview

Configuring interworking

The Interworking page is used to configure whether or not the VCS acts as a gateway between SIP
and H.323 calls.
To go to the Interworking page:

•

VCS configuration > Protocols > Interworking

To configure interworking using the CLI:

•

xConfiguration Interworking Mode

H.323 <-> SIP interworking mode

The options for this setting are:
Off: the VCS will not act as a SIP-H.323 gateway.
RegisteredOnly: the VCS will act as a SIP-H.323 gateway but only if at least one of the endpoints is
locally registered.
On: the VCS will act as a SIP-H.323 gateway regardless of whether the endpoints are locally
registered.

You are recommended to leave this setting as RegisteredOnly (where calls are
interworked only if at least one of the endpoints is locally registered). Unless your network
is correctly configured, setting it to On (where all calls can be interworked) may result in

unnecessary interworking, for example where a call between two H.323 endpoints is made over
SIP, or vice versa.

Enabling SIP endpoints to dial H.323 numbers

SIP endpoints can only make calls in the form of URIs - e.g. name@domain. If the caller does not
specify a domain when placing the call, the SIP endpoint will automatically append its own domain
to the number that is dialed. So if you dial 123 from a SIP endpoint, the search will be placed for
123@domain. If the H.323 endpoint being dialed is just registered as 123, then the VCS won't be
able to locate the alias 123@domain and the call will fail.
The solutions are to either:
1. Ensure all your endpoints, both H.323 and SIP, register with an alias in the form name@domain.
2. Create a pre-search transform on the VCS that strips the @domain portion of the alias for those

URIs that are in the form of number@domain.
Refer to the

Pre-search transforms

section for information on how to configure pre-search

transforms, and to the

Stripping @domain for dialing to H.323 numbers

section for an example

of how to do this.