// All diagrams

Interactive · 14 steps
How a SIP call works
Step through a complete SIP call — INVITE, 100 Trying, 180 Ringing, 200 OK, ACK, RTP audio, and BYE — with real SIP messages at each stage.
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Interactive · 8 steps
How a mobile call works
From radio signal to cell tower, MSC, HLR lookup, carrier interconnect, and destination phone — the full journey of a mobile call.
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Animated comparison
VoIP vs PSTN
Side by side — circuit-switched PSTN vs packet-switched VoIP. See how they differ in routing, cost, quality and vulnerability.
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Interactive · 6 steps
How SMS works
SMS-SUBMIT, SMSC store and forward, SMS-DELIVER, delivery reports — the complete journey of a text message including what happens when they're offline.
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Interactive · 5 steps
How 1800 toll-free numbers work
SCP number translation, reverse billing, geographic and time-based routing — why you call for free and who actually pays.
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Security awareness · 5 steps
How call spoofing works
Why scammers can fake any caller ID — the SS7 vulnerability, how spoofed numbers flow through the PSTN, and how to protect yourself.
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