There are a number of issues that can contribute to VoLTE call drops. Coverage gaps often occur due to changes in topology or neighbor cell interference. Visibility to the SIP messages between the Call Session Control Function and the Telephony Application Server can provide valuable information about current and future RAN coverage needs.
Phased upgrade activities can result in mismatched software loads and incompatible handover situations between eNodeBs. Examination of failure cause messages between the PGW and PCRF will help to isolate problematic MME tear down of associated bearers.