In GSM it isn’t possible to make a call with Inter PLMN handover. One of the reasons is that you must select the new service provider. Thera are other reasons too.
Yes it is possible. In the Visiting MSC the external lac along with the MCC and MNC is defined. Also u have to define the MAPMSC and MAPVLR for the MSCs between which u want to perform the handover. It is possible. SIEMENS is supportinmg this feature from sr11