A user reports that several IP phones in a single department are displaying a continuous "registering" message. Which fault domain should be investigated?

A router fault would typically affect a broader segment of the network beyond a single department, not isolate the issue to one group of phones.

A CUCM subscriber node failure would affect phone registration across multiple departments or the entire cluster, not just one localized department.

Phones within a single department typically connect to the same access-layer switch, so a switch failure, a misconfigured VLAN, or a spanning-tree issue would simultaneously disrupt network reachability for all phones on that switch. This prevents them from completing CUCM registration, causing a persistent 'registering' state across the affected department while phones elsewhere remain functional.

A hardware or firmware fault on individual IP phones would typically affect one phone at a time, not all phones in an entire department simultaneously.

A SIP gateway is involved in outbound PSTN call routing, not in phone registration to CUCM, so its failure would not cause a 'registering' message.