QoS (quality of service)

Voice and video packets must arrive on time; a file transfer only needs to arrive eventually. QoS lets switches, routers and firewalls tell the difference, usually by marking packets with a differentiated services code (DSCP, defined in RFC 2474) and then queueing marked traffic ahead of the rest. It helps most on the links that are actually congested: the uplink to the internet and Wi-Fi.

For a growing business QoS is the difference between a phone system that sounds fine on day one and one that still sounds fine when the team doubles and someone starts a large cloud backup at noon. It is configuration, not hardware, so it is often skipped, and the symptoms (choppy audio, frozen video) get blamed on the provider.

Ask whoever manages your network whether voice is marked and prioritized end to end, including on the firewall and Wi-Fi, whether your internet provider honors or strips the markings, and how they verify it. Ask your phone provider which markings their phones and apps use so the two configurations agree.