An SMS firewall is the mobile operator's filter that checks every business SMS message arriving at its network, and blocks the ones that should not reach phones. It blocks scam messages, messages from senders who have not registered, and business messages that try to enter the network by a cheap, unofficial route.
Operators run SMS firewalls for two reasons. The first is to protect their customers. Fake SMS messages are one of the commonest forms of phishing, a fake message built to trick a person into giving away a password or money. The second reason is money. Operators charge a higher rate for A2P messages, the messages that businesses send to people, than for messages between people. Some senders try to avoid that rate.
The firewall applies three checks to each message as it arrives.
The first check is the sender. In India, every business must register its sender name, called a header, under TRAI's rules. A business message from an unregistered header is blocked.
The second check is the wording. Each business also registers the fixed wording of its messages, called templates. The firewall checks that the message matches one of that sender's templates. It also checks any web link in the message against the links that the business has registered.
The third check is the route. Say a company wants to send lakhs of promotional messages cheaply. Instead of using an A2P SMS aggregator, it buys hundreds of ordinary prepaid SIM cards and puts them in a machine called a SIM box. The SIM box sends the messages as if people were sending them. This is called a grey route. The firewall notices that one SIM card is sending 3,000 almost identical messages an hour. No person types that fast. So the firewall blocks that SIM card's messages and reports it.
For instance, consider a message that arrives at an operator's network, addressed to thousands of customers in Bhopal: "Your electricity connection will be cut tonight. Call this number now." It comes from an ordinary mobile number, not a registered header. Its text matches messages that the firewall has already seen from hundreds of other numbers that evening. The firewall blocks every copy before it reaches a phone.
The firewall sits inside the operator's network, beside its SMS centre and its packet core, the system that checks each SIM card and connects it to the network. It passes its records of grey-route traffic to the operator's revenue assurance and fraud management team, which tracks money that the operator should have been paid.
An SMS firewall is different from a spam blocking app on your phone. The firewall stops a message inside the operator's network, before it reaches any phone. A spam blocking app works on your phone, after the message or call has arrived.