Hi Nagendra, please let me know what issues you are experiencing.
Several issues (handovers, insufficient capacity, lack of Radio signalling, resources, etc..) can lead to call dropping.
The Call Flows are well established with 3GPP (includes GSM). To reduce on
interference for GSM, Frequency Hopping can be ued. For 3G, Frequency Hopping is part of WCDMA.
VBR/ Wallis Dudhnath