On average, every day, about 50,000 minutes of traffic (calls) are terminated in each country. This means that terminators’ VoIP GSM gateways process over 16,000 calls a day. To keep track of this flow, the GSM-operators develop special AntiFraud systems that block phone numbers that make a suspiciously large number of calls, with operators investing millions of dollars to create these programs...more interesting information you can find here: https://goantifraud.com/en/blog/basic-principles-of-telecommunications/