We have received a few reports of intermittent call failure when using Telstra SIP Connect. Only a handful of our customers have Telstra SIP Connect trunks and only a couple have a failover point in case of network problems so it has taken a while for this fault to manifest itself to an annoying level for our customers.
The symptom is that intermittently a call will fail and go to the failover point although the trunk appears to be functioning correctly, registered and there are no Internet problems.
This could have turned into a real head scratcher in view of what the problem turns out to be but fortunately the cause has already been identified and the fix is very simple.
The complexity in troubleshooting lies in that the issue only manifests itself from a caller with an iPhone that has been upgraded to iOS 11. An Invite is sent out by the network to the phone system and because it is larger than the Avaya expects, the call is rejected. It does not seem to occur with any other providers.
The fix is simply to change the transport type in the Line programming. This change fixes the problem and does not appear to have any other side effects.