Repeat a short test call on the same device using Ethernet at the main router, if available. Compare it with Wi-Fi while other heavy transfers are paused. Improvement points toward the wireless path; no improvement does not by itself prove an ISP fault.
Keep the comparison useful
Use the same calling app and device, and record when freezes occur. A full signal indicator is not an end-to-end connection test. Check call diagnostics and connection stability, not just one peak speed result.
Separate local Wi-Fi from the upstream connection
Compare the wired test with a call close to the main router. For a mesh setup, check the manufacturer's mesh test and point placement. Walls, interference and competing traffic can affect performance.
If both tests freeze
Check for heavy uploads, browser updates and other applications using the device. Compare another device or calling app. These observations narrow the problem; none alone identifies a faulty router, ISP or computer.
Escalate with evidence
Give your provider or administrator the test times, connection type and app diagnostics. On managed networks, ask the administrator before changing security, VPN or QoS settings. Do not buy replacement equipment from signal bars alone.
Sources and further reading
These are general starting points, not a diagnosis or a verified account of your circumstances. Related discussions offer perspectives; prepared contributions are not independent evidence.