Routing international calls over the internet into a bank of local SIM cards so they terminate as cheap local mobile calls, bypassing official interconnect rates.
A SIM box, or GSM gateway, is a device that holds many SIM cards and can place calls on a mobile network as if they were ordinary subscribers. Fraudsters carry international traffic over IP to the box in the destination country, then terminate it as local mobile-to-mobile calls, avoiding the higher international termination rate the mobile operator would normally charge.
The mobile operator loses termination revenue, and in many countries the government loses taxes or surcharges built into international rates. Call quality also tends to suffer, and the recipient often sees a local mobile number instead of the original caller ID. Consumers may also lose trust in caller ID when they repeatedly see unfamiliar local numbers for calls that are really coming from abroad.
Detection usually relies on spotting SIMs that behave unlike real people: very high outbound call counts, almost no inbound calls or messages, little movement between cell sites, and calls that line up with known bypass routes in test-call campaigns. Operators in affected markets often run regular test calls across international routes specifically to find which routes are delivering traffic through bypass equipment.
Why it matters to carriers
For wholesale carriers, the risk is being part of the chain that delivers bypass traffic, which can damage relationships with destination operators and lead to disputes or blocked routes. Understanding which of your vendors terminate through legitimate interconnects, and testing routes for caller ID substitution, helps you avoid unknowingly selling bypass termination.