Connection test
Measures what your calls will experience from this network. Nothing to install, no sign-in; about two minutes. Keep the page in the foreground while it runs.
Latency, jitter and loss
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Round trip to the support server. Voice wants under 150 ms, jitter under 20 ms, loss under 0.5%.
Download
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Each call needs about 100 kbps; this shows the headroom around it.
Upload
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Upload is what carries your voice out. It is usually the tighter side.
Firewall and NAT
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Checks that UDP can leave your network and how the firewall presents you to the outside. More than one public address points at load-balancing or symmetric NAT, which phones dislike.
Phone platform
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From our side: latency to the platform and a live registration of a test extension. Confirms the platform itself is healthy right now.
SIP ALG check waiting for probe
A browser can't send SIP, so this last check needs one command run on a computer inside the same network as the phones. It sends two test packets to us and prints the answer; nothing is installed.
SIP ALG ("SIP helper", "SIP transformations") is a firewall feature that rewrites phone traffic and is the most common cause of one-way audio and phones dropping registration. The check above proves whether it's on.