In short
An SMS grey route is a delivery path that exploits an interconnection, technical configuration or commercial arrangement in a way that allows A2P traffic to reach subscribers without using the operator's intended commercial A2P route. The message is delivered; the termination fee is not paid.
Grey, not black
The terminology matters. A black route would be outright illegal. A grey route usually is not: it typically uses real interconnects, real agreements and real protocols, but applies them to traffic they were never intended or priced for. A P2P interconnect agreed for subscriber messaging carries an enterprise's OTPs; a roaming agreement carries commercial traffic that should have been terminated under an A2P contract.
That ambiguity is exactly why grey routes survive. Each individual element can be defended, and the party benefiting is frequently several steps upstream from the operator that notices the effect.
How they are constructed
Most grey routes are built from one of a handful of techniques, often combined:
- Injection of commercial traffic over P2P interconnects priced for subscriber messaging
- SIM box or SIM farm termination, so A2P messages appear as ordinary MO traffic from local subscribers
- Delivery from unauthorised Global Titles, or from partners with no A2P agreement
- Direct injection into the SMSC or an SMPP endpoint that lacks proper validation
- SMS home routing bypass, so inbound traffic never passes the screening point
- Sender ID misdeclaration, so commercial traffic is classified as something cheaper
What it costs beyond the fee
The obvious cost is the unpaid termination revenue. The less obvious costs are usually larger. Grey routes commonly alter Sender IDs, truncate or modify content, break multipart messages and add latency — so a bank's OTP arrives late, from an unrecognised number, or not at all.
The subscriber blames the operator. The bank blames the operator. And because the traffic is invisible in the wholesale records, nobody in the commercial team can explain why complaints are rising while A2P volumes look flat.
Why they come back
Bypass operations rotate. MSISDN ranges are replaced, Global Titles change, Sender IDs are adjusted, and supplier relationships are re-papered through a different intermediary. A static blacklist decays within weeks.
Durable control comes from behavioural detection that recognises the pattern rather than the identifier, combined with periodic route testing that confirms what is actually arriving at handsets on the destination network.