Thanks Alex & Olle!

I've done some tests with Keepalived for Virtual IP failover, and I found that it works more or less as you mentioned, Olle: if an Invite comes from Server #1, and that server gets offline, the BYE will indeed be correctly processed by Server #2. 

Isn't DMQ supposed to handle this? as i understand, DMQ sends the SIP messages to other servers, so wouldn't it be enough? (I assume, I haven't tested DMQ yet.... )