It starts with a detailed Root Cause Analysis of this issue or issues that you are focusing on. As an example you mentioned “Called Part Busy” and we need to see is this Network Busy (lack of Network resources) or is Party B busy and on a call.
First port of call is to look at the Network setup and see how the Network is performing during the “Busy Hour” period or periods. This will help to see if you need more capacity in your Network. Does the Signalling, Switches, IP/MPLE(-TE / -TP), Radio Network, etc.. need further capacity. Next step is to look at what Software Features are activated and see if any Licenses will need to be increased, etc.. Basically, an Audit will be required and this will help a plan of action to be formulated. In the past I experienced major issues with call setup. From the Audit it pointed to issues with the Transmission / Optical Network. A capacity expansion (MPLS increase) helped to remedy the issue.
RF planning may also be required to ensure that you have available resources in “congested” areas.
Look forward to your response.
VBR/ Wallis Dudhnath