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If you mean your using a Vodafone UK Payg SIM card and wanting to roam there then you'll need the roaming bar lifted which you can do in your myvodafone or use customer services on 191 or Live Chat.
The costs can be found in Vodafone UK and travelling-abroad.
Your phone would need to support the frequencies and band's used by that countries network too.
If a person is using a SIM card with the brand name that is not Vodafone UK then they'll need to contact their own countries Vodafone customer service as they are are not linked even tho they use the same Vodafone brand name.