then return the call. When the service calls back, the user is being charged for the cost of the service from the credit available in the home network. The service will then prompt the user to enter the dialed number of the party to be called. The disadvantage of this method of roaming is that the user will not be able to dial numbers directly from the handset. The advantage is that it works in almost all locations around the world since USSD is ubiquitous and free. The user can direct dial from their handset if the network they are roaming in supports CAMEL (Customized Applications for Mobile networks Enhanced Logic). This allows real time billing by the home operator without having to dial the customer back. The advantage is that it is more natural and works seamlessly. The disadvantage is that not all networks support CAMEL so the list of countries where a prepaid customer can use their phone abroad is smaller than for postpaid mobile phones. Privacy rights Since July 2013, at least 80 countries globally (including 37 on the African continent) have mandated, or are actively considering mandating, the registration of prepaid SIM users. As of January 2020, researches found that governments of 155 countries have been mandating SIM registration laws; in some countries, prepaid users must submit biometrics like photos and fingerprints. The reason is a concern by police and security agencies that anonymous use of prepaid mobile services facilitates criminal or terrorist activities. Prepaid phone users can be anonymous for two reasons: In certain countries, the prepaid SIM card can be sold in a shop like any other goods. There is no need to register them at point of sale, unlike postpaid phones who have to credit check the user before allowing them to purchase and enter into a contract. As prepaid services can often be topped up using cash and vouchers, there is no way to trace the payment and thence determine the identity of a prepaid phone user from payment records. While there is no doubt that criminals and terrorists use prepaid SIM cards to help stay anonymous and avoid easy detection, to date there has been no empirical evidence to indicate that: Mandating the registration of prepaid SIM users leads to a reduction in criminal activities. The lack of any registration of prepaid SIM users is linked to a greater risk of