1. This problem can be there if ciphering is enabled in Siemens and disabled in Alcatel MSC.So handover from Alcatel to siemens is failing whereas reverse is possible.
2. Problem in definition of outer cell at Alcatel MSC. May be that there is no definition for outer (neighbouring) cells in Alcatel MSC for cells catered by Seimens MSC.
3. Both MSCs are working on different MAP version.