What is a SIM box? How does it work?

Coline Constant
19 Jul, 2023

It’s a fact that for several years now, with the rise of teleworking and the development of so-called global communication, cases of fraud have been on the increase. And in the telecom sector, SIM boxes are at the top of the list of frauds—for both users and telecom companies. But what is a SIM box and how does it work? What are the consequences of its use, and what are the ways to detect and, above all, protect against it? Let’s take a look at the situation.

What is a SIM box?

A SIM box, also known as a GSM Gateway, is a device used to route SMS messages via a prepaid SIM card, or calls via a VoIP (Voice over Internet Protocol) gateway installation. This object contains several SIM cards, linked to the gateway in the case of calls, but housed and stored separately. The box can contain SIM cards from different mobile operators, and therefore work with several GSM gateways in different locations.

How does a SIM box work?

The general principle of the SIM box is to allow users to use local and/or Internet SIM cards and the VoIP gateway to reach the recipient of their communication.

Let’s see how it works in practice for SMS and calls.

  1. Inserting SIM cards in the SIM box

The point of the SIM box is to put in several SIM cards. This is logical. Each SIM card inserted in the box is associated with a unique telephone number. And it is these SIM cards that the box uses to send SMS and make calls.

  1. Configuring the SIM box

It is possible to configure the SIM box with specialized software.

For calls, once the cards have been installed, the SIM box needs to be connected to the Internet, via a broadband connection, to be able to make VoIP calls.

  1. Sending communications

When the user sends an SMS from the SIM box, the device software makes a request to send the SMS via one of the prepaid SIM cards inserted.

Calls are made via VoIP gateways. These devices convert VoIP signals into traditional telephone signals, routing outgoing calls to the public telephone network. So when a user makes an outgoing call, the SIM box intercepts the call and selects a free SIM card to make it.

  1. Making outgoing communications via SIM cards

The message is then transmitted to the SIM card operator’s local mobile network and routed to the operator’s SMS message center (SMSC), which temporarily stores the message until it is delivered to the recipient.

The call is redirected to the network as a local call.

  1. Communication delivery and reception

The recipient’s SMSC receives the message and attempts to deliver it to the recipient’s cell phone. The recipient receives the message on his or her cell phone as if it had been sent directly from another cell phone, from a number that appears to be local.

Calls are made via the network, as a local call, and appear to come from a legitimate phone number.

What are the risks and consequences of using a SIM box?

In principle, a SIM box can be useful, that’s for sure. But in reality, it’s a dangerous system, and one that borders on the legal (if not completely illegal). In any case, whatever your objective, you need to be aware of the risks and consequences of using a SIM box.  

Here are just a few of them:

Against operator contracts

Today, telephone operators specify in their contracts that it is forbidden to make commercial and/or automated use of the unlimited SMS messages included in their offers. In the event of detection or suspicion of use of a SIM box, they reserve the right to block the suspect card. In other words, from one day to the next, the card(s) can be blocked. It’s a legitimate way of protecting yourself and limiting the use of these devices. In fact, given the “SIM box hunt”, such a system usually only works for a few weeks, if you are lucky. It is then detected and blocked. Would you take that risk?

Slow service

Using a SIM box means sending a large number of communications. But with a conventional SIM card, a user can only send a maximum of 2 messages per second. Imagine the time it would take to send several thousand bulk SMS messages, even with several SIM cards… Clearly, this is of no interest to you or your business. Not to mention the risk of lawsuits…

For automated mass communications (SMS or calls), it’s best to turn to professionals like RingRing. Our business is to ensure that yours can communicate efficiently, professionally and securely. First, we have agreements with telephone operators to ensure quality delivery of your communications. We can also send a very large number of SMS messages per second, to guarantee the speed of your communications. And we own and operate secure short numbers, which we make available to our customers, to ensure the continuity and security of your communications.

Lack of longevity

The reputation and status of SIM boxes make them a system you can’t rely on over time. By working with this kind of installation, you are in no way guaranteeing the longevity of your business and your communications. That’s right, given the operators’ hunt for SIM boxes, any card used in such a system can be deactivated at any time. As a result, another card (and therefore another number) will be used for communications. How can you be recognized by customers if your number is constantly changing? How can you create a climate of trust? Clearly, this is not at all a reliable system for developing a legal and flourishing business.

How to detect a SIM box?

SIM boxes can be dangerous, but it’s essential to be able to detect them to protect against illegal use.

There are several ways to detect them. Here are 3 of them, one of which you can do yourself:  

Checking phone numbers and invoices

When you receive an international communication, SMS or call, the correspondent’s number can give you an indication of its origin.

SIM boxes tend to use numbers that look like local cell phone numbers but have international prefixes or access codes.

In addition, for SMS, mass communications and/or commercial purposes are carried out via short numbers, owned by recognized companies working directly with operators, such as RingRing. As for long numbers, if they are used in a regulated way, they will start, in Belgium, with “+32 460 2…”.

So, if you suspect you are receiving SMS or calls from a SIM box, you can check the correspondent’s number. This will give you a good indication of the origin of the communication.

Traffic data analysis

On another level, mobile operators can analyze traffic data to detect SMS and calls made via SIM boxes. SMS and international calls routed via these may have different traffic patterns to normal communications, which can help to identify them.

Use of specialized detection equipment

There is also specialized equipment designed to detect SIM boxes. They can be used to monitor cell phone networks and detect messages and calls from these.

It is important to note that detecting these installations can be difficult. Their users can configure them to bypass traditional detection methods. Mobile operators are constantly working to improve their detection techniques, but they are not infallible.

The SIM box is a system that can be used for fraudulent telephony. To protect yourself, it’s important to understand what a SIM box is, how it works and the potential consequences for users and telephone operators. Fortunately, there are ways to detect and protect against fraud.

Do you have any questions? Don’t hesitate to contact us!

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