Hi,for your starting point–>
>Trunk Utilization would be better less than 75% – Dual Seizure and Congestion should all be near zero.
Check ABR in total and on interfaces – compare BID and accepted BID, It should be near 100%, one reason which can decrease ABR is Dual seizure it contributes low ASR in core side
Top 15 cause codes – check release code then you can find abnormal release which can affect ASR
Paging success rate.PSR should more than 98%
Signaling link load
Packet loss at MGW (For qualitative VOIP)
Be aware prepaid subscriber would potentially decrease ASR due to Money issues ( check abnormal release code)