[Users] call drop in a few seconds

[unplug]

When UA1 calls UA2, UA1 drop the call before UA2 take the call.  After
a few second, UA1 makes another call to UA3.  The call established and
it will drop after a few second.  I expect the situation below.

UA1 ------INVITE ----> UA2
		UA2 (ringing)
UA1 ------CANCEL----> UA2
                            UA2 (ringing)
UA1 -----INVITE--------> UA3  (in few seconds)
UA1 ------call established----UA3
UA1 <-------BYE------- UA2
UA1 -----call drop--------UA3

What can I do to reduce the situation above?