Most SIP phones are "multi-presence". They can handle multiple calls. As a result, they never go "busy", or at least, not until all of their presences are in use. That's why you get the 10s ring on the second call; the phone is using its second presence. On some phones (e.g. Snom), you can set the phones into single presence mode, in which case they will go "busy" and use the busy message.
Most users use the open and closed trunk routing to handle open/closed messages and Vmail routing.
Kind Regards
S