Content Comparison

Symptom

An E1 ISDN line on a Dialogic IMG Integrated Media Gateway connected to British Telecom (BT) is able to receive inbound calls but attempting to make an outbound call can fail with a cause code relating to the number being invalid or incorrect.
 

Analysis

The following traces were obtained from a telnet session with the IMG 1010. By entering ‘Advanced Debug’ mode (d) from the main menu, then ‘ISDN Diags’ (I) and ‘2->06->ff’ we enabled a raw Q.931 trace on all E1s. The trace is disabled with ‘2->00’. The same commands can be used on the IMG 1004 and IMG 2020. 

A successful inbound call:

...

From looking at the above, we can see that in the failure case the call is being rejected by the network because the number we dialled was unallocated or unassigned. Comparing the dialled number in the successful case against that of the failure case, we can see that the ‘type of number’ is set differently, i.e., ‘national’ in the failure call from the IMG and ‘000 - unknown’ in the successful call from BT.

By default, the IMG will pass whatever Nature of Address Indicator (NAI) it sees on the inbound call leg out to the outbound call leg. In the above case, the NAI is set to ‘national’ for the inbound leg and this will therefore be propagated to the outbound leg.

A solution is to change the behaviour on the IMG for this E1 by forcing the NAI for outbound calls over this E1 to ‘000 - unknown’.

Fix / Solution

Note: Screenshots in this article have been taken from an IMG 1010 but equally apply to the IMG 2020, though some features might look differently on the IMG 2020.
A new outgoing translation table to the routing configuration may need to be added if one does not already exist. Right-click on the ‘Routing Configuration’ node and select ‘New Outgoing Translation Tables’ to add the ‘Outgoing Translation Tables’ group. Then right-click on ‘Outgoing Translation Tables’ and select ‘New Translation Table’ to add a new table:

...