A grey route is an SMS delivery path that reaches an operator's subscribers without a proper commercial agreement, so it is cheaper per send but unpaid-for and unreliable.
A grey route is a messaging path that carries application-to-person SMS into an operator's network without an official termination agreement, so the operator is not paid for delivery. Because there is no contract behind it, a grey route is cheaper per message and is the usual source of a below-market tariff — but delivery is inconsistent and often silently dropped by the operator's SMS firewall. Mobilesquared estimated grey routes still carried around a quarter of global A2P SMS traffic in 2022.
For OTP and payment authentication, grey-route weakness shows up as failed first-attempt delivery, more billable retries, and abandoned transactions — which is why the cheapest quote frequently produces the highest cost per delivered OTP. Operators fight grey routes with firewalls, so a path that delivers acceptably one month can be filtered the next, making the effective delivery rate drift over time. Direct, agreement-backed routes cost more per send but land more codes on the first attempt.
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You pay per SMS sent, but only delivered OTPs work. A worked example shows why the cheapest vendor costs more.
Bulk SMS alone leaks budget. Verify the list, then run WhatsApp-first with SMS fallback.
