Topic
#telephony
3 posts
- What happens when you press 5 on a phone call A key press is two sine waves. On a modern VoIP call it is usually not sound at all, but a small packet that says which key and for how long. A walk from the 1960 Bell Labs design to RFC 4733, with the codec test that drops digits. 9 min read #systems#telephony#voip
- NAT bought us IPv4. It made real-time audio weird. NAT let the internet outgrow its public IPv4 address supply. It also made a connected VoIP call a surprisingly different problem from a working one. 8 min read #nat#voip#sip
- The phone number was flagged before we ever used it Recycled phone numbers carry the previous owner's spam reputation. We linked out to Truecaller instead of showing our own guess, which meant reverse-engineering an undocumented Android deep link with adb. 2 min read #reputation#telephony#android