Telephony and conferencing

Voice is the service where failure is most visible and least tolerated. Nobody files a ticket when email is slow; everyone notices a dead phone, and customers notice first. It is also the service most likely to be nobody's stated job — inherited from a facilities vendor, half-migrated to a collaboration suite, with the parts that still work unowned.

Voice is an application on the network now, and behaves like one

A phone system is real-time traffic sharing a network built for everything else. That makes voice the first service to expose problems the network was hiding: jitter, packet loss, and asymmetric routing that a file transfer absorbs and a conversation does not.

Quality complaints are almost never about the phone system. Diagnose them as network problems on the path, and prioritize voice traffic explicitly rather than assuming capacity is enough — a link with headroom on average still queues at the moment someone is talking. See it-operations:network-administration for the diagnosis order.