Roaming and settlement is the software that lets an operator's customers use a foreign operator's network while abroad, and later works out how much each operator owes the other. Roaming means using another operator's network with your own SIM and number. Settlement means paying the money that the operators owe each other. In India, most roaming is international roaming, because each large Indian operator covers the whole country with its own network.
Before roaming can work, two operators sign a roaming agreement. It sets the wholesale price that the foreign operator charges the home operator for each minute, SMS and megabyte. An Indian operator may hold such agreements with several hundred operators around the world.
Say a family from Chandigarh flies to Dubai, in the United Arab Emirates, for a wedding. The father has bought an international roaming pack of ₹2,999 for ten days. When the plane lands, he switches on his phone, and it looks for a network. A local operator's network finds an Indian SIM. Its core sends a request to his operator in India, through a roaming exchange, to ask whether this SIM may roam. His operator's packet core checks that the SIM has a roaming pack, and approves the request. His phone now works on the local network, with his own number.
For ten days, the local operator records every call, SMS and megabyte that he uses. Every few hours, it sends these records, in a standard file format, to a data clearing house. A clearing house is a company that sits between many operators and handles their records. It checks the records, including the wholesale price that the local operator has put on each one under the agreement. Then it forwards them to his operator in India. His operator then deducts the use from his pack, through telecom billing and real-time charging.
At the end of each month, a financial clearing house adds up what every operator owes every other operator. Indian operators also serve foreign visitors to India, so money is owed in both directions. Only the difference is paid. This is called netting.
Mistakes here cost real money. A record that never arrives is use that is never billed. A SIM stolen and taken abroad can run up large charges within hours, before the records arrive. So roaming records are also checked by revenue assurance and fraud management.
The same clearing companies often carry text messages between operators in different countries, for example an OTP sent from an Indian bank to a customer abroad. Within India, business messages reach the operators through A2P SMS aggregators. All of this is part of the operator's business software, in telecom OSS and BSS.