In mobile networks, when the mobile terminal (MT) has an active call while crossing the boundary of two systems, it is required that the existing connection shouldn?t be lost. To meet this demand, the signalling protocol of mobility management must be designed for supporting location registration and call delivery for roaming users who move beyond their home network. The objective of this paper is to provide a modified Boundary Location Register (MBLR) signalling protocol for Inter-system roaming. The location registration and call delivery process of the new protocol are proposed which tries to reduce the signalling cost and latency. An analytical model is developed for comparative performance analysis of the BLR and MBLR protocol. However, it is realize that this will result in increased processing requirements at the database server. Thus, tradeoff between signalling, database access costs & processing overheads must be recognized.