An end user reports that conference calls are failing. Which fault domain should be investigated first?

An IP phone problem would typically affect only that individual user's calls, not all conference calls system-wide.

CUCM Media Resources host the software and hardware conference bridges required for multi-party calls. When conference calls fail broadly, the most likely cause is that no conference bridge is available in the Media Resource Group List (MRGL) assigned to the relevant device pool. Without an available conference bridge, CUCM cannot allocate the resources needed to join multiple parties.

Voice gateways handle PSTN connectivity and are not responsible for the internal conference resource allocation required for multi-party calls.

Network infrastructure issues would cause broader call failures across all call types, not failures isolated to conference calls.