There is no single best channel. SMS reaches nearly any phone, WhatsApp suits users who live in the app, voice helps when text fails, and email works when the user is already at a desk.
How they differ
- SMS: widest reach, needs no data plan, but exposed to SIM swap and SS7 interception.
- WhatsApp: needs data and the app, usually richer and cheaper per message in some markets, with fixed template text.
- Voice: slower and costlier, but works on feature phones and for users who cannot read well.
- Email: low cost, but only as secure as the inbox and often slow to open on mobile.
A practical rule
Pick the channel your users already check within seconds. Then add a second channel for the cases where the first one fails, rather than hunting for one perfect channel.
Tambua supports all four, plus M-PESA and Reverse OTP, so you can compare them on real traffic with free test keys.
Need OTPs that reach users across Africa? Read the API docs or get a free test key.