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TravelSim is a brand-name of Top Connect OÜ, Estonia based telecommunications company headquartered in Tallinn. TravelSim is a Prepaid GSM SIM card that provides telecommunications to clients in 190 countries.
Callback service means that when users dial an international number using a TravelSim prepaid SIM card, the company picks up the call, routes it to the number dialed and calls back, connecting the two ends of the line.
In 2014 Top Connect announced that the TravelSim service had saved its user base a collective $200 million in roaming charges in 2013.