SIM box fraud, also called bypass fraud, is when international calls are diverted away from the official interconnect and delivered through devices loaded with local SIM cards, so they reach the called party as cheap local calls. It deprives operators and governments of termination revenue, hurts call quality and caller ID, and is illegal in many countries, so wholesale buyers should learn to recognise and avoid such routes.
What a SIM box route is
International calls into a country normally arrive at the destination operator through its international gateway, which charges the official termination rate. A SIM box (also called a GSM gateway) holds many local SIM cards. Traffic sent over the internet to the box comes out as local mobile calls, paid at retail or bundle prices far below the international rate. The person called sees a local mobile number instead of the original caller ID.
The difference between the two prices is the bypass margin. That is also why bypass is common in countries where international termination rates are high compared with local call prices.
Why it is a problem for buyers too
- Caller ID is lost: the called party sees a random local number. Your CLI route is not really a CLI route.
- Quality is poor: calls pass through compressed internet links and radio, causing echo, delay and drops, see MOS, jitter and packet loss.
- Routes disappear: operators detect and deactivate the SIMs, so ASR collapses without warning.
- Legal and reputational risk: in many countries bypass is a criminal or regulatory offence, and regulators and operators act against the equipment and the parties involved.
- Unlicensed traffic: some destinations require international calls to pass through licensed gateways; a bypass route breaks that rule regardless of who sent the call.
How to recognise a bypass route
| Sign | What to check |
|---|---|
| Price well below what the destination operators charge for international termination | Compare with other offers for the same breakout |
| Received caller ID is a local mobile number that changes between calls | Run a CLI delivery test |
| "Local CLI" offered for a country where it is not commonly available legally | Ask how local numbers are assigned; see local CLI routes |
| Limited channels, unstable capacity, frequent outages | SIMs being blocked and replaced |
| Poor audio, background noise, long PDD | Radio and internet legs in the path |
| Supplier vague about how the route reaches the operator | Ask whether it is direct, via the international gateway or via another carrier |
How operators detect SIM boxes
Operators use two main methods, often together:
- Test call generation: test calls are placed from abroad through many wholesale routes to SIMs the operator controls. If a call arrives showing a local number, the SIM that delivered it is identified and blocked.
- CDR and network analysis: SIMs used in boxes behave differently from people. Typical signs are many outgoing calls and few incoming ones, almost no SMS or data, calls to many different numbers, a fixed location, and one device rotating through many SIMs.
Many countries also require SIM registration and limit bulk SIM sales to make bypass harder.
How to protect your business
- Ask suppliers in writing how each route reaches the destination and whether it uses licensed gateways.
- Test CLI on every new route and periodically after go-live.
- Treat prices that look too good as a question, not a bargain. The factors behind normal prices are explained in what drives VoIP termination prices.
- Include in your agreements that routes must comply with the law of the destination country, with the right to stop traffic without penalty if they do not. See the interconnect agreement checklist.
VoIPMatcher sources VoIP termination from its carriers and asks how each route is delivered. If you need a destination with genuine CLI, request a CLI route.
Frequently asked questions
What is a SIM box used for?
A SIM box, or GSM gateway, holds many SIM cards and turns calls arriving over IP into mobile calls. In some countries companies may use one to route their own staff calls to mobiles. Used to terminate international traffic as local calls, past the official gateway, it becomes bypass fraud, which is illegal in many countries.
Is a SIM box illegal?
Owning a GSM gateway is allowed in some countries and restricted in others, but using SIM boxes to carry international calls past the official gateway is illegal or a regulatory offence in many countries. Regulators and operators block the SIMs, seize equipment and act against the parties involved. Wholesale buyers should avoid routes that depend on them.
What is the difference between SIM box fraud and grey routes?
SIM box bypass is the most common form of grey voice route. A grey route is any path that avoids the official interconnect and its termination rate; SIM boxes do it by converting international calls into local mobile calls. Grey SMS routes use the same idea, with SIM farms sending business messages as person to person texts.
Why are SIM box routes cheaper?
Because they pay local retail or bundle prices instead of the official international termination rate, which in some countries is many times higher. That gap is the bypass margin. A CLI price far below what legitimate operators charge for the breakout is therefore one of the clearest warning signs of a bypass route.
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