How verification works
The mechanics behind OTPs, VoIP checks, carriers and number marketplaces.
VoIP vs non-VoIP numbers for verificationHow apps tell a VoIP number from a carrier line, why they care, which services refuse VoIP, and how the catalog labels physical-SIM and virtual pools.
How SMS verification worksWhat happens between tapping send code and the SMS appearing: OTP generator, SMS gateway, carrier routing, short codes, 10DLC, and where temporary numbers fit.