SMS engineering notes

Notes from operating SMS routes, by smsroute.cc.

• Why Your 160-Character SMS Became 3 Messages: GSM-7, UCS-2, and the Segmentation Math Nobody Reads
• Exactly-Once Is a Lie: Building Idempotent SMS Sending with Retries That Don't Double-Text Your Users
• Your SMS Says "Sent" but Never Arrived: A Field Guide to DLRs, SMPP Status Codes, and Carrier Filtering
• Parsing Phone Numbers Is Harder Than Parsing Dates: E.164, libphonenumber, and the Inputs That Break You
• Someone Is Draining Your SMS Budget: Rate-Limiting OTP Endpoints Against Enumeration and SMS Pumping
• Your OTP resend button is a race condition (and it's charging you twice)
• "Delivered" doesn't mean delivered: reading SMS status callbacks honestly

Open data

Sender ID rules by country — dataset on GitHub (CC-BY-4.0).