 This typically means that incoming calls are free, no matter which network a mobile user is on.This also means that a caller enjoys the lowest cost when making a call, significantly reducing call costs, especially compared to normal network charges for International Roaming.Global roaming SIMs are very often combined with callback technology, whereby the user dials a number in the normal way, but the call is intercepted by an application on the SIM card and turned from an outbound call to an inbound call which the user answers.