An A2P SMS aggregator is a company that carries businesses' SMS messages to the mobile operators over direct connections. A2P stands for application-to-person: a message that a company's software sends to a person, such as a bank alert, an OTP or a bill reminder. In India, most of these messages pass through a small number of Indian aggregators.
A bank could, in principle, connect to every operator itself. In practice, it would need separate connections to Jio, Airtel, Vi and BSNL, each with its own technical setup, contract and prices. The aggregator has already built these connections. Each one is a dedicated link between the aggregator's servers and the operator's SMS centre, the operator's system that stores SMS messages and delivers them to phones. A business connects once to the aggregator, and reaches every operator.
Consider a cooperative bank in Kolhapur that sends an SMS alert after every withdrawal. A customer takes ₹5,000 from an automated teller machine at 7:42 p.m. The bank's core banking system sends the alert text and the customer's number to the aggregator. The aggregator then does three things. First, it finds the customer's operator. The first digits of the number are not enough, because people keep their number when they switch operators. So the aggregator looks the number up in a list of switched numbers. Second, it passes the message over its connection to that operator. Third, it waits for the delivery report, the operator's note that the SMS has reached the phone, and passes it back to the bank.
India adds one more step. Under TRAI's rules, every business must register its sender name, called a header, and the fixed wording of each kind of message, called a template. These are recorded on the operators' DLT platform. DLT stands for distributed ledger technology, a shared record that every operator can read. The bank's template might read "Rs {#var#} debited from a/c XX{#var#} on {#var#}", where each {#var#} marks a part that changes. If a message does not match a registered template, the operator's SMS firewall blocks it. Aggregators help their clients register headers and templates, and find out why a message was blocked. Alerts and OTPs may be sent at any hour, but promotional messages may be delivered only between 10 a.m. and 9 p.m.
Aggregators also receive Indian traffic from global messaging API platforms, which hand over the last step to them. OTP verification services send their codes through them too. Their growth now comes less from SMS and more from the WhatsApp Business API and RCS, which they also sell. Many businesses reach them through a WhatsApp marketing platform. How operators check and price these messages is explained in A2P messaging.