 Using a normal network locked SIM, travellers for instance, can use their own mobile phone (if it's enabled for roaming), in any country that has a roaming agreement with their home network, or for global networks like Vodafone, with another Vodafone OpCo.This manifests itself to most users when they receive a text message welcoming the traveller to a local network.However, once they return home their SIM will only work on the network with which they have a contract.