RADVision SCOPIA 400 User Manual page 171

If a routing method fails, the Gateway automatically tries to route the call
through the next routing method in line. If all methods fail, the call is rejected.
The call might also be rejected if the Gateway routes the call to an endpoint that
is busy or not available.
Table 4-1 explains the routing methods.
Table 4-1 Routing Methods
Routing Method Explanation
DID The Gateway supports two forms of DID (Direct Inward Dialing):
Multiple Subscriber Network (MSN) and sub-addressing.
About Dialing In to the IP Network via the Gateway
MSN—The telephone company assigns a group of
phone numbers to a particular ISDN line by the
telephone company. PRI ISDN lines are usually
assigned multiple numbers in the US and in Europe.
When MSN is used, an ISDN terminal or phone can dial
directly to an IP network endpoint. The call is still
routed through the Gateway but the Gateway is
transparent to the person dialing from an ISDN terminal.
An H.323 endpoint on the IP network registers with the
gatekeeper using one of the MSN numbers. When an
ISDN terminal dials the MSN number, the call routes
through the Gateway ISDN port connected to the line
with the MSN service to the endpoint that registered
using the requested number.
Sub-addressing (PRI Gateways only)—Sub-addressing
is a one-stage DID dialing mechanism in which a phone
sends two numbers. One number is for routing on the
circuit switched network. The other number is
forwarded to the Gateway inside a Q.931
sub-addressing information element for IP address
resolution by the gatekeeper.
Sub-addressing can also be used for implementing
ISDN fallback when not enough bandwidth is available
for routing an IP-oriented call over IP. Implementing
ISDN fallback requires the support of the gatekeeper.
Using the SCOPIA Gateway 161