SIP 302 is a 'Moved Temporarily' response from a SIP server to tell the client (the phone) to go and register against a different SIP server. This is useful to help direct phones to the correct SIP server and to help load balance servers and so on.
The Message is defined in RFC 3261, but unfortunately not all SIP clients support it.
FireBrick SIP supports this, both as a server and a client.
Use Within the AAISP VoIP Platform
AAISP was going to use feature to enable the service to be load-balanced across multiple SIP servers, however, as support for this was limited/broken in many devices it is not being used. AAISP are still balancing traffic over multiple servers.