A call uses several network services
A phone may need address assignment, DNS, accurate time and a provisioning path before it can register. SIP signaling then establishes the call, while media commonly uses separate traffic. Record these dependencies instead of treating all phone traffic as one port.
Start at the boundary that failed
If a device has no usable address, inspect cabling, power and address assignment. If registration fails, examine name resolution, reachability and authentication. If a call connects but sound fails, investigate negotiated media addresses and packet delivery in each direction.
Compare working and failing paths. An internal call that succeeds while an outside call has one-way sound provides a useful boundary: the endpoint itself may be capable of audio, while the carrier-facing media path differs.
Capacity is more than a speed test
Voice depends on timely packet delivery, not only advertised link speed. Upload saturation, queueing, wireless contention and packet loss can hurt calls even on a fast connection. Measure during a busy period and preserve direction-specific evidence.
QoS can prioritize traffic on equipment that honors the policy. It cannot guarantee how an unrelated internet network treats your packets. Reserve headroom, document the bottleneck and validate improvements using actual call behavior.
Sources & applicability
Primary references for the technical details above. Operational examples and planning checklists are VoIP.info editorial guidance.