Appearance Contention Resolution|
Two UAs could try to use the same appearance number for an outgoing request
Before UA uses appearance number, it is sent (PUBLISH or NOTIFY) to Appearance Agent
Appearance Agent decides who wins
Winning UA uses appearance number after it gets NOTIFY from Appearance Agent.
Loosing UA will get a 500 response with Retry-After from Appearance Agent.
Receipt of a 500 to a NOTIFY is problematic
Use PUBLISH instead.