We have Two PBX - A primary which handles all traffic and is linked to a registration based sip trunk to an external provider. The second PBX is purely there to give a route out to a second trunk.
We have just had new firewalls installed but when we pull the plug on the primary internet line the PBX does not seem to detect this and follow the trunk sequence to the backup. It sits for around 15-20seconds attempting the primary but then times out with Cause 18. It doesn’t try the backup route which is next in the trunk
Would this be because the trunk is registration based and we would need to wait for the 3600 sec timer to expire before it moved on?
Why would it not attempt next in sequence with a cause 18?
The firewall installer has tried everything they can think of, hours of testing but we can’t get the system to attempt the second route
Assuming that the primary trunk is pjsip with default settings, Asterisk sends an OPTIONS (qualify) request every minute and if there is no response after three retries, the trunk should be marked unreachable. So if the connection has been down for more than ~90 seconds, an outbound call should not even try the primary trunk and should go straight to the backup.
Having a fallback is essentially unworkable without qualify. With normal SIP setting, status 408 (cause 18) won’t be reported until after 32 seconds, by which time many callers will have given up.
Possibly, setting Expiration to 120 seconds (or the shortest that the provider will accept) will meet your needs.
If not, consider running a script that checks network connectivity (for example, pinging one hop before the provider’s address) and enables or disables the trunk accordingly.