An entire department is reporting frequent calls with poor voice quality. Which fault domain should be investigated first?

An IP phone hardware or configuration issue would typically affect only that individual user, not an entire department simultaneously.

CUCM media resources such as transcoders and MTP resources are involved only in specific call scenarios (codec mismatches, SRTP) and would not cause widespread departmental voice degradation.

SIP trunk problems affect calls traversing that trunk to or from external destinations, not the internal voice quality experienced by an entire department on local or on-net calls.

Network routers and switches are the common infrastructure shared by all phones in a department, and issues such as interface congestion, missing QoS policies, or excessive jitter/packet loss at that layer would degrade voice quality for every affected user at once, making this the most logical first investigation point.