This is something that I'm interested in as well.
This is my case. In the normal flow Party A, calls and is connected to party B. If however, when dialing party B,
1) invalid number - mark B as being invalid so he's not tried again, then call party C and try to connect with A
2) number not reachable - mark B as being offline so he's not tried again in the near future, then call party C and try to connect with A
3) gateway down - Retry dialing B with an external Sip gateway, and mark the default gateway as down, triggering a notification to the server admin
Ben, whats required to add these error codes to the Rayo protocol, then to Adhearsion?