Hi, anh
MS2 probably have active diverts.
So in this situation scenario is:
MS1 calls, MSC sends Paging to MS2 but MS2 at the same moment does cell reselection so he doesn’t “hear” paging. Call is devirted. As you see in Disconnect message, call have not failed, it have Normal call clearing so MS1 just ended call when he heard Voice mail or some IVR answer.
This is normal case, especially if MS2 is using dual mode because some phones very often does cell reselection between 2G and 3G so chances of missed call for them are high.