A given scenario:
Opensips distributing calls to a number of pstn gateways. One of them goes away and t_relay fakes 408 internally. This is checked for in on_reply, marks the gateway as failed and tries another.
However if a call returnes with isdn cause code 18, this is also mapped to 408 - and can cause the gateway to be marked as failed. If every gateway is tried with the same result, everyone will be pulled from the list.
I would be nice to be able to use for example 503 as this response code too.